View Full Version : use FILLERS when applying epoxy filler coat (2nd, 3rd epoxy coat) to fill weave

06-27-2014, 04:25 PM
When sheathing plywood boat with fiberglass cloth and epoxy, do you use FILLERS when applying epoxy filler coat (2nd, 3rd epoxy coat) to fill weave of cloth ?? fillers = micro-ballons, fumed silica,

06-27-2014, 04:36 PM
You can, but you don't have to.

06-27-2014, 04:47 PM
Hmm...after you property apply and saturate the cloth , any additional epoxy is non structural, parasitic weight.

To fill the weave means many things....for above waterline ,non structural sheathing , like a cabin top you can apply fairing compound directly to the epoxy glass sheathing to fill its weave

for structural work.....areas in which secondary bonding will be needed , you fill the weave with a slurry of epoxy mixed with filler...any structural filler will work...no micro balloons.. This filler coat is then sanded to key in the secondary bond.

a slurry would have half as much filler as your fillet mix. Slurry is many times squeegeed on. Roller or brush will work but messy.

for underwater work that needs maximum water resistance, you roll on one coat of straight epoxy, one coat of slurry epoxy to fill the weave...then fair the bottom in with filler...then coat the sanded filler with and addition coat of epoxy .

when filling weave always work wet on wet. ...impregnate the cloth...wait till the green stage...fill the weave.

dont allow the epoxy to cure before filling.

Never sand the glass epoxy sheating untill the fibers are burried under resin, slurry or fairing compound. If you sand...cut the fibers...you ruin the structural properties of your laminate.

06-27-2014, 05:28 PM
I used straight epoxy wet on wet (two coats on vertical or angled surfaces, on the flat one seemed to hold thicker and didn't need a second). Epoxy with filler was only used for fairing hollows.


06-27-2014, 05:38 PM
On a sail-n-oar boat, does one need to do the cloth/epoxy sheathing on the bottom, or can simply a few coats of neat epoxy on plywood followed by paint be sufficient?

06-27-2014, 05:55 PM
I like WEST 407 for this, although I have used 410 microlight on waveboards in my past, where weight savings were measured by the oz. :d Once again slug pretty much nailed it from my point of view. / Jim

Todd Bradshaw
06-27-2014, 07:09 PM
Plain epoxy tends to be far more abrasion resistant than the lightweight, balloon-based fillers (407, 410, etc.) if you think you might hit anything or beach the boat. It can still scratch, but nowhere near as deeply. As long as you wash any blush or other contaminants off the hull first, you can actually fill with no sanding needed several days after the initial resin application and get a perfectly good chemical bond (something that in itself tends to be highly over-rated).

Rather than counting filler coats (which will vary greatly in thickness when applied by different folks with different tools and in differing conditions) it is much wiser to fill with thin coats (prevents runs and sags) until the cloth texture totally disappears (no matter how many coats it takes) and then add one more for a bit of a sanding cushion - so that you can eventually sand the surface smooth without cutting the cloth. Then give it a week or so to fully cure before sanding. Otherwise, you're still working with partially cured resin and risking the health consequences (not very smart).

06-27-2014, 07:31 PM
+1 for Todds answer.

Fillers are fine if you are going to paint.
Use high strength fillers to avoid the abrasion Todd talks about.
Since it will be stronger, it will be tougher to sand out smooth :D
Graphite powder seems to work out well for abrasion, if you will accept an uneven gray or you are going to paint (but the paint will be easier to scratch than the epoxy).

06-27-2014, 10:33 PM
Same old argument ....hard, harder, hardest. Yes, straight epoxy is a harder finish than the using of 407, and the hardest part is the multiple thin applications on vertical surfaces, which incidentally usually need less abrasion resistance imho, and subsequent sanding, a week later? Not gonna happen for me. I overlap 6 oz cloth when I do layups and using 407 to handle the cloth edges with minimal sanding, works better for me. slug handled the description of a scratch resistant finish re. his underwater section description, finishing with a straight epoxy layer. Hi-build primer, epoxy or otherwise, over that.

As to chemical bonds, let's say I am great fan of post-curing, or any version thereof. Use of fillers begats a painted finish, for sure. :d Fillers that are tough to sand will mess up your day, with little advantage to the paint you will lay over it. As before, each to their own. / Jim

06-28-2014, 07:14 PM
Thanks for the information. It is very helpful. What I actually did: PAST 1) I applied 6 oz cloth, wait and let it cure over night. 2) Sand with 60 grit. 3) applied 6 oz X 4 inch tape to chine, 4) let it cure over night and sand the next morning. 5) applied epoxy with small amount of brown Phenolic Micro-Balloons to fiberglass covered bottom using a foam roller to fill weave. This roller left 100s, maybe 1000s, of bubble marks in epoxy. FUTURE I will let it cure for 2 days then sand with 60 grit. Apply straight epoxy with foam roller and use 4 inch chip brush to break bubbles and smooth epoxy. In the future 1) I will apply epoxy to "green" epoxy to save myself a lot of time, 2) do much less sanding, 3) wait longer for epoxy to cure.

Todd Bradshaw
06-28-2014, 11:26 PM
Sanding epoxy a day or two after applying it (or less) is about the best way possible to get sensitized to it and make sure that you can never use it again, aside from just smearing it on your skin for fun. If you learn to squeegee your cloth properly and roll and tip your filler coats, none of that sanding between coats will ever be needed - even in transitions where you are going from one layer to two or more. The filler coats themselves will flood the vast majority of the step-downs and when you have let it cure properly, they will smooth out without an awful lot of sanding as you do the rest of the hull - and it shouldn't take more than 80 grit to do the job. I have done graduated stem-bottom "grunch patches" with as many as six or seven layers of ten ounce cloth transitioning down to one or two layers on the main part of the hull this way with no sanding between layers. It's simply not that hard to do and there is no good reason to expose yourself to green resin dust.

Two other things that can be a problem in some applications are that sanding green fiberglass/epoxy can fracture a lot of fibers, down inside the laminate. It isn't likely to do significant structural damage, but if you happen to be clear finishing, like on a strip canoe, these tiny fractures show as small white fibers and there is no way to get rid of them. The speed of some sanders (big disks in particular) will also sometimes generate enough heat to actually soften and smear epoxy resin. If you're trying to get as much mileage as possible out of your disks, and maybe using them a bit past their prime as they fill up, they generate excessive heat even easier. The more time the resin has had to truly cure, the better it will resist being damaged.