View Full Version : Cold Molding and Vacuum Bagging

Bob Perkins
11-17-2003, 12:04 PM
Hi Everyone,

I'm moving right along with my project and expect to have my first layer of plywood completed in the next couple of weeks.

I'd like to vacuum bag the remaining layers for a few reasons, hopefully some weight savings, better bonding between layers, less staple pulling, and it would be fun.

So I'm doing a lot of research and wonder if anyone has any good tips, etc..

1) Where to pick up used vacuum pumps? (I'm watching ebay - but I'm not 100% sure what to look for. Are there any used pump places near Boston?

2) Any good cold molding sites or references to vacuum bagging of this type.

3) Personal experiences doing this?

4) What not to do...

I realize this is rather vague... It's because I don't know what I don't know yet. Google has been a big help so far


Bruce Hooke
11-17-2003, 01:04 PM
Both WoodenBoat and Fine Woodworking have published some excellent articles on this so I would start by digging up those articles. They have excellent recommendations on the capacity necessary and many other issues. The usual boat building approach of using a single sheet of plastic sealed down around the edge is a bit different from the usual furniture approach which uses a sealed bag that completely encloses the object.

I stumbled on my vacuum pump used at a camp up in Maine where it had been used to start the water system(!) so I can't offer any recommendations on sources. I will say that in my limited experience volume is a bigger issue than absolute vacuum pulled. Especially with the boatbuilding method of using a single sheet of plastic sealed around the edges leaks can be hard to completely eliminate, so you need enough volume to keep up.

You will sometimes come across references to using Mortite or Dum-Dum to seal down the edges of the sheet of plastic. If you go this route don't get the stuff at the hardware store -- it is useless. Someone told me that a much better version is available at auto parts stores. I never investigated further because I had already switched to using a single sealed bag that I close with wood strips and clamps -- but I could do this because I was not doing a whole boat.

Good, heavy gauge plastic is availble from McMaster-Carr. I made up my own bag from plastic purchased there and it cost a fraction of what finished bags go for.

11-17-2003, 02:11 PM
bought a reasanable good (centrifugal)vacuum pump from an old milking machine, these have almost everything you need vacuum gauge, a safety valve, and a reservoir if accidently (milk) resin is sucked up

kind regards,

Jack C
11-17-2003, 04:49 PM
Check out Kurt Hughes multihull site (http://www.multihulldesigns.com/pdf/cm33.pdf). He uses a shop vac. I kid you not.


11-17-2003, 05:03 PM
sorry not a centrifugal vacuum pump, it is a vane pump .there are firms who do the service of these perhaps you could contact them?

11-17-2003, 05:11 PM
The West System pamphlet is a good start.

For a project like yours, the tough part is in getting a tight enclosure. I don't know how to do it in something that complex and bony on the inside. A couple of pinhole-sized leaks will kill you. It's imperative to dry-run to see if you can get a tight bag.

Ebay is a good source of vacuum pumps. It's a bit of a pain to sort through the manufacturer's data to figure out what's being offered. For large items like yours, i'd look for a 10 CFM pump. Wide-open volume is important to you. Ultimate vacuum is not. I recently scored a Gast 3/4 HP 10 CFM pump for $140, including shipping.

The other consumables are easily available:
Peel ply from Aircaft Spruce or www.peelply.com (http://www.peelply.com)
For the bleeder-breather layer, polyester batt from the yardgoods store works fine and is inexpensive.
4 mil polyethylene film fron the local building supply does the job if you don't ask it to stretch. Nylon film is tougher.
vacuum bag tape is available fron Aircraft Spruce. I've used hardware store 'dumdum' or even duct tape. Whatever works.

It's good to have help. I'm near boston and eager. Phone is 781 237 7564.


11-17-2003, 07:10 PM
Big things to watch while vacuum bagging are:

Bridging, the part on top is now actually in contact with the piece underneath, with a resulting void underneath. Also bridging in the bag, you need to carefully place folds in the bag, leaving enough material so a bridge does not form.

Sharp edges which will puncture the bag.

Marking sure that the part is not crushed by the vacuum, at 14 pound per square the force exerted is great. Donítí try to vacuum bag the whole hull at once.

Not sucking the resin into the pump! You need enough of a lead in the vacuum lines.

[ 11-17-2003, 07:12 PM: Message edited by: brian.cunningham ]

Bob Perkins
11-17-2003, 08:38 PM

Thanks for your replies - You are all validating some of the concerns that I have.

For those who are interested, I found this site: Vacuum Press (http://www.joewoodworker.com/veneering/welcome.htm)

Their design is geared toward the doit yourselfers like us..

On Vacation
11-17-2003, 09:23 PM
Well, maybe John will translate this Oyster language, since he is fluent in Oyster, most of the time, but here goes, for now.

For the novice and low cost laminating hulls, especially on this straight foward design of sides and bottom, rip strips of plywood, 1 1/2" wide, 1/4" and 3/8" using either scrap or full pieces of whatever type you have, and lay off to the side for now.

I assume you are using at least one more layers of laminate under the topcoat layer,? Will you be finishing the hull bright? This hull is custom made for strips of plywood for compression glue laminates. Take the next pieces, dry fit them, then do your glue up, with staples on the ends of the convex curves, and staples in the middle of the inverted or concave curves of the strips, to hold them in place.

Put the strips of plywood, depending on the extreme compound curves, flat, on the layers of laminates, long ways the running length of the hull, sometimes cutting the strips into shorter lengths, and use flat washers and sheetrock, fine thread screws, run through the plywood, and new laminates, to compress and spread the load, with less hardware, removable with ease, and glue up sections, of pieces, or complete sides or bottoms, making sure if you do this in sections, to you clean the edge of the piece you will stop at, of uncured glue. Use the 3/8" strips in the stern or aft part and the 1/4" inche in the vee area, and the foward sides.

A hull of this size, will require about three main runs, and depending on how it lays, some small pieces between the longer ones, turning these on an angle to longitual.

Sometimes, in the foward flares and on the vee area of the foward bottom, this will require small pieces on a diagonal, to fit the compound of the contour of the flare.

On the last layer, if running long ways, of solid wood, you can use these small strips, running from chine to sheer area, especially in the convex area, or the aft, or tumblehone section, with 3/8" or even 1/2" plywood if good quality, if you have semi uniform pieces of veneer, to minimize the amount of staples needed for the last job. If you don't then when using this method, check for voids and use some small wedges between the strips and laminates to push them into the surface under the new pieces.
Undertaking a first time project of doing a full hull, for the first time, is well, not worth the chance for the one shot that you get, for the uncured glue, and the time fitting of the wood, and material, if you screw up.

You can back the pieces of plywood up with the packing tape, so when it will not stick to the glue. Notch trowel your glue. If you are using cotton fibers, make sure you lubricate the screws with some form of oil, and remove them, when you are confident that its is cured, and not three to five days later. Use only fine thread black screws. Cabosil is usually the best filler, for the job.

Try to fair each layer, before applying the next layer. You will be happier with the end result of the topcoat.

Make sure, if you have some rough finish woods, you only use short sections of the plywood strips for compression. Back your staples up with a heavy cardboard for ease of removal and less marks into the top layer of wood.

If I didn't make myself clear, than.......

[ 11-17-2003, 09:35 PM: Message edited by: Oyster ]

John Bell
11-17-2003, 09:30 PM
Well, maybe John will translate this Oyster language, since he is fluent in Oyster, most of the time, but here goes, for now. :D

You done good, my friend.

11-21-2003, 10:18 PM
Well done Oyster!

I only have one thing to add that might be of use.
See if you can get your hands on some plastic straping material. The Post Office uses alot of it and throws it out. It the stuff they use to band box's of letters with.
Using that between the surface and shooting staples thru it.
1] The epoxy will not stick to it.
2] It gives you a tail to grab a hold of so you can get the staples out easier and not have to dig into the wood.

11-26-2003, 11:00 PM
A good place to look for a used vacuum pump cheap is your local university chemistry lab. the vacuum pumps the generally use are the oil bath type (Welch), and they are run continuously in most cases. When they wear out the grad students usually in charge of maintaining the labs generally do not realize that rebuild kits are far less expensive than new pumps and they are ssimply thrown away. Check the dumpster loading dock outside the chem labs.

11-27-2003, 12:04 AM
I have read of using old refrigeration pumps - see '10 woodenboats you can build', as published by the venrable WB mag, pg. 94. it also says to use a vacuum gauge purchased at an autoparts store to keep tabs of the vacuum pressure - to detect leaks.

Keep us up to date with what you're doing; I am really interested in the results, and good luck smile.gif

edited to add:
the strakes on the bottom of your boat (top of the picture) seem to sag off the keel to the chine, is this the design or the picture fooling my useless eyes? It looks like there is a 'hollow' all along the keel. Is this part of the design and can anyone tell me why it would be designed like that for the entire length along the keel or is it all perspective?

b.t.w. what design is that? I love the tumblehome aft...

[ 11-27-2003, 12:10 AM: Message edited by: Braam Berrub ]

08-02-2013, 09:54 PM
i just got my kit from http://www.veneersupplies.com/ aka joewoodworker.com "V2 with the auto-cycling air venturi DIY" and will build my little system soon! I liked this kit because it uses an air compressor instead of a vacuum pump, this is cheaper (if you have an air compressor) and also has less moving parts to service or wear out. The kit was onsale and after a few upgrades only cost about 185$ though one needs to buy a few things like vacuum resivor-- aka an old or new air tank from harbor freight, and a few switches and wiring boxes and what nots. along with the standard bagging, peelply, and sealing tape...

i dont have any personal work with this yet but should soon... ill report back after i make a few skateboards for the kiddos as a warmup for using it on my cold molded boat hull...

i know that it is time sensitive and shouldnt be done without precut, prefit, and trial dry run.
also i know that several times i have read that you have to seal the crap out of the first layer (to prevent leaks) and should let it sit for a full week before trying to vacuum bag over it.

heat garage as most epoxy needs around 60F or higher to setup right...

08-03-2013, 05:31 AM
So I'm doing a lot of research and wonder if anyone has any good tips, etc..

1) Where to pick up used vacuum pumps? (I'm watching ebay - but I'm not 100% sure what to look for. Are there any used pump places near Boston?

One place you can check is your local Air Conditioning & Refrigeration services. They sometimes have used vacuum pumps that are good enough for a hobbyist. I picked up my Robinair 5 CFM pump for under $100


You can see it here with the Joewoodworker.com vacuum controller. It's a little more involved than the venturi generator mentioned previously but it's intended for larger jobs and can be left unattended to manage a vacuum pull.

I also have a venturi vacuum generator, Gast, that's portable for smaller jobs.


The downside of the venturi generator is it's need for a high volume supply of compressed air. Depending on how long the job will take the compressor will have to run for quite a long time. The Joewoodworker vacuum controller above will shut itself down when it reaches its set level and will automatically restart when vacuum drops.

2) Any good cold molding sites or references to vacuum bagging of this type.

I learned a lot from this site.


I wrote about my experience on Composites Central a few years back.


3) Personal experiences doing this?

I've done a few projects, mostly vacuum infusion but bagging follows the same principals. Here's a small layup.


Most of what you see, the film, mastic tape, tubing, vacuum reservoir & resin trap are all assembled with materials from Lowes.

4) What not to do...

Do not spill epoxy on your basement floor then drag an old carpet over it so the Wife won't see the mess! :o

I don't know how critical it is with vacuum bagging but with infusion you can pull too much vacuum and end up with a dry part. Bridging, which has been mentioned is an issue. Use plenty of pleats in your bag so you don't pull up the edges when drawing down a vacuum. Always use a resin trap.

I realize this is rather vague... It's because I don't know what I don't know yet. Google has been a big help so far

Good luck


trent hink
08-03-2013, 06:50 AM
H Rick,

The mastic tape you get from lowes, what is it sold as? A brand name might help because the search feature at lowes.com is a seriously bad joke.


08-03-2013, 07:31 AM
I think I got it in the window & door section Trent. I don't know if it had a brand name. That was 5 or 6 years ago I got it. If not try a camper trailer place. They use it all the time for sealing windows in campers.

08-03-2013, 07:50 AM
Swimming pool companies have really good vacuum pumps that they discard regularly for some reason (Maybe they can't handle the super heavy material of swimming pool liners eventually). I got one for $10 from a company that was going to throw it in the dumpster.
WAY too big for what I needed it for, but I gave it to a guy who worked at Brooklyn Boat Yard, and last I heard they still used it for cold molding small boats.
Vinyl shower curtain material (Available from fabric stores, or online) makes great bag material.
The foam tape you can buy as weatherstripping makes good seal stuff. Use PVC cement to seal large pieces of shower curtain material together.
2 pieces of PVC pipe, one split lengthwise, make excellent bag closure clamps.

08-03-2013, 10:45 AM
4 mil poly film from the paint dept. at the big box is OK bag material. Look out for wrinkles.

I recollect a sticky strip weatherstripping that was just OK. I prefer the purpose-made vacuum tape from Airtech Online (https://store.airtechintl.com/htm/defaultnetscape.htm).

I have not felt the need for the reservoir and regulator setup on my pump. Instead, the pump runs nonstop and there's just a vacuum gauge and a valve to bleed in ambient air to regulate the level of vacuum. Simple enough.

08-04-2013, 08:18 PM
Hey Bob,

Lots of good information for you here, hopefully by now you have found the West Systems guide to Vacuum Bagging:

I went with a Venturi system and got a compressor as I'd like to do some spray finishing in the future and buying a standalone vacuum device wasn't that much cheaper and wouldn't allow me that flexibility to run other air tools etc. You can buy a kit from most marine dealers that uses a west systems venturi to generate your vacuum, or you go with a company like Piab or one of those, if you search venturi on fleabay your bound to see a few. As long as you understand the basic principals of vacuum, your golden.

You can never evacuate more vacuum than you have air pressure at your location, so if your at sea level and you have

Barometric Pressure:
1015.0 hPa

which is the current pressure where I live, that's the maximum clamping force a perfect vacuum could achieve on your bag. (perfect vacuum is almost unattainable, especially on the cheap). Fortunately you don't need a perfect vacuum, but the better the seals the less work your compressor will do in maintaining that vacuum.

You can't create more clamping pressure than the weather at your location allows...

Anyway good luck to you, I'm only new to all this fun stuff myself and had a failure last night after what i think at this stage was some epoxy starvation, i should have pre coated my veneers before putting them in the bag. (working on a model boat)

08-10-2013, 07:54 AM
Using stretchlon bag material can lessen pleats, and make more complex curves easy...

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