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model14
03-31-2013, 11:34 AM
In painting over an epoxy coating, either epoxy over wood, or epoxy in fiberglass cloth, do you normally use a primer prior to applying an enamel color coat? I am looking at using a quality one part marine enamel for my color coats, inside and out. (I am building the 14 foot Spira Seneca.)
Thanks,
Richard

Todd Bradshaw
03-31-2013, 12:39 PM
Sanded epoxy itself is a better primer for enamel than just about anything else you can put on there. About the only reason to prime it would be to fill surface imperfections with something very soft and easy to sand. In general though, you're probably better off just making sure that your epoxy surface itself is smooth, clean and ready to go without needing primer.

jim_cricket
03-31-2013, 01:23 PM
I understand what Todd is saying, but I almost always use an appropriate primer and sand it out. It lets me see just how good the surface is before going on to the top coat. I'm probably a fussier finisher than most though. The single part polys like Interlux or Petit (my latest favorite for their color pallete) are quite good, flow out well if thinned a little with their thinner, and have excellent gloss retention over the long term, particularly if the sun is kept off them with a cover.

http://jimluton.com/images/mde_primeweb
primer on epoxy-

http://jimluton.com/images/deckfinish1_web
finish paint- petit easypoxy

I think Todd is right that you don't need it for adhesion, as long as the epoxy is WELL cured, and sanded.
Cricket

wizbang 13
03-31-2013, 01:26 PM
I almost always use a 2 part epoxy primer , and specifically , almost always interlux 404/414

Peter Malcolm Jardine
03-31-2013, 01:57 PM
I almost always use a 2 part epoxy primer , and specifically , almost always interlux 404/414

+1.

Bob Cleek
03-31-2013, 04:56 PM
While the two products can sometimes be interchangeable, there is a difference in purpose between "primer" (which goes on bare wood) and "undercoat" or "basecoat," which goes on beneath finish coats. If your surface is as smooth as a baby's bottom after priming, and the color of your workpiece is absolutely uniform, you should be good to go for building up finish coats. If not, you are wasting the higher priced finish coating if you are trying to build up a solid color (especially white) with finish coatings. Base or undercoat contains a much higher amount of pigment, often talc-like material in sanding undercoatings, which covers well and is intended to do so. Gloss finish coatings contain a lower level of pigment and don't cover as well. It can often be nearly impossible (well, six or eight coats) to obtain a uniform white with gloss finish coatings applied over a mottled surface. Don't waste time and material trying to get a good finish coat without a perfectly smooth and perfectly uniform colored surface. If your primer achieved that, so much the better. Otherwise, a sanding undercoat coating is required.

Peter Malcolm Jardine
03-31-2013, 05:00 PM
What Bob has said is the reason I like 404. It is an epoxy, so it has all of the admirable qualities of that substance, such as waterproofing and adhesion characteristics, but it is also a high build, and allow you to deal with very small defects. It is really good for covering minor spidercracking in gelcoat as well. It comes in white or gray, but you can tint the white, which helps with coverage on a finish coat.

paul oman
03-31-2013, 06:05 PM
on very rare occasions the nonyl phenol in most epoxies (usually used to make mix ratios whole numbers) can make alkyd enamels not dry for days or weeks. An odd combination of temps humidity etc. Never something you can reproduce... Most folks have experienced once or twice. Often blamed on amine blush.

So while it happens very rarely, I like to prime. For one reason the primer has a uniform color that shows flaws in the epoxy coat you will miss otherwise. Also, the primer gives a uniform color instead of a mix of colors and textures with epoxy/putty/epoxy/wood surface.

I like to use an aluminum pigmented mcu coating called aluthane. It has the advantage of turning from light gray to dark gray when sanded - this is quickly shows high and low spots on the surface. In most situations you cannot beat an mcu coating for bond adhesion etc. I know of no marine supply store that sells mcu coatings.

that said with the exception of the sometimes nonyl problem, epoxy is generally considered a universal primer. It is almost a requirement under any 2 part LPU poly coating.

Also since epoxies take about a week to cure, it is generally better to topcoat them within a few days - this then provides some chemical bond between the not fully cured epoxy and the topcoat in addition to the ordinary mechanical bond between dry surfaces.

paul - progressive epoxy polymers inc

skaraborgcraft
04-01-2013, 03:09 AM
Interesting. I have just stripped off the oil based paint below the waterline on my Michalak Skiff, after too much of it had blistered and peeled after being afloat for 3 months. I had used a water based primer over the glassed epoxy bottom, as a guide for smoothness between getting all the seam tapes filled and fair and once smooth,it had a thin coat of the waterbased primer and was then overcoated with the oil based gloss. Interesting thing to me is that the waterbased primer and waterbased fine surface filler was still intact under the peeled oil paint. As the finish paint was reccomended and not particulary cheap,i cant say i was amused to be stripping the bottom off back to the epoxy coating. I was intending to use an aluminium primer and leave it like that.

Bob Cleek
04-01-2013, 01:23 PM
Interesting. I have just stripped off the oil based paint below the waterline on my Michalak Skiff, after too much of it had blistered and peeled after being afloat for 3 months. I had used a water based primer over the glassed epoxy bottom, as a guide for smoothness between getting all the seam tapes filled and fair and once smooth,it had a thin coat of the waterbased primer and was then overcoated with the oil based gloss. Interesting thing to me is that the waterbased primer and waterbased fine surface filler was still intact under the peeled oil paint. As the finish paint was reccomended and not particulary cheap,i cant say i was amused to be stripping the bottom off back to the epoxy coating. I was intending to use an aluminium primer and leave it like that.

Not a surprising result, given the use of an oil based finish coat over a waterbased primer, no?

Todd Bradshaw
04-01-2013, 05:10 PM
I haven't had very good luck with oil-based stuff over water-based stuff (and I'm not talking about the rubbery, latex-types of water-based stuff). In one case, I accidentally grabbed the wrong can of varnish and put a refresher coat of oil-based Varathane over water-based Varathane on one of our oak floors. I taped out the plan for a sail and the tape peeled the oil-based varnish off cleanly. To remove it without making a mess sanding, we actually duct-taped the entire floor and just peeled it all off. It should be noted that many enamels, including those made for marine use (Brightside, Easypoxy, etc.) will not tolerate long-term immersion (sometimes as short as a week or less) without peeling. So it may or may not have had anything to do with the paint/primer relationship. Always read the can, and if it says "not for long-term immersion" believe it.

It should also be noted that some of the products containing aluminum flake or powder may not do so well under paint. The aluminum is popular with barrier coating formulas, for example, because it not only increases the coating's hardness to some extent, but also increases moisture resistance. Moisture literally has to zig-zag around the tiny aluminum flakes to penetrate through the coating. However, when you sand these coatings as your final paint prep you are exposing tiny bits of raw aluminum, the edges and surfaces of these particles. Raw aluminum begins to oxidize immediately upon exposure to air and paint and other coatings usually won't bond well to aluminum oxide. You may get a lot of tiny spots where your paint covered it up, but isn't very well attached, and you may even have a surface covered with a whole lot of tiny, pin-point-sized fisheyes, where the aluminum repelled the paint leaving a bare spot. I did one two-tone boat a while back where one color of the paint was thick enough to bridge these spots, and the other color had less pigment or filler and didn't. All the teeny-tiny fisheyes didn't really hurt anything, but after about six months, I couldn't stand looking at them any more and sanded it off, sealed it with a coat of resin, sanded that smooth and repainted.

skaraborgcraft
04-02-2013, 03:12 AM
Cant say i have had issues with waterbased primer and then a top oil coat, at least above the waterline, but obviously this particular oil paint was not suitable for the job. Nothing on the can was translated into not for use below the waterline, and i took the sellers words as correct,maybe that was my mistake. My canoe has the same waterbased and a different brand of oil paint over,but perhaps its only used for a few hours and then is out of the water makes the difference,that paint jobs 5 years old and intact.

The aluminium primer wont be painted over. Its used on a fresh water lake, that supplies the surrounding inhabitants with their drinking water,so good anti-foul is frowned on. The aluminium paint seems a bit more resilient to fouling.

skaraborgcraft
04-02-2013, 03:14 AM
Not a surprising result, given the use of an oil based finish coat over a waterbased primer, no?

Care to explain why? Surprised me.

model14
04-02-2013, 03:39 PM
Thanks everyone for the very useful information. I haven't decided if I will use primer or not, but your inputs have given me a lot to work with in the decision process.
Richard