Sealer under polyester grain filler

Why must a catalyzed urethane be used as the sealer under polyester, instead of other materials? June 24, 2001

I'm playing with polyester grain filler. Manufacturers' literature recommends using a catalyzed urethane as the sealer under polyester, since polyester and oil-based stains don't do well together. What's wrong with using either a catalyzed vinyl sealer or conversion varnish as the sealer underneath the polyester? Several manufacturers of polyester grain filler allow the use of a conversion varnish as a topcoat. If it's okay above the polyester, what's wrong with using it underneath the polyester, as well as a sealer? The C-V should be able to act as an excellent barrier between the polyester and the oil stain.

Forum Responses
From contributor R:
Polyester manufacturers have put together time-tested systems that work. Not all products work together. I know that all CV products use acid catalyst, and some can take up to three weeks to dry completely. As this finish is drying, it is gassing formaldehyde, and this can cause polyester to change color and can keep it from adhering well. You don't know what other things may go wrong. It's best to stay with a system that you know will work.

I'll use my old "bottom of the drum" conversion varnishes as a wash coat, or better as a barrier coat, before using polyester primers. I wouldn't do this on clears for the reasons mentioned above. With most poly systems, you can use their barrier coat, let it sit for 3-4 hours and go right over with polyester sealer without sanding. It really doesn't get easier than that because you don't have to worry about burning off the edges. I never had any problems with this system other than the fact that once opened, the urethane barrier (part B) converter goes bad quickly. So only buy in gallons, not 5's, and pour off the unused converter into smaller containers and fill them right to the top.

From the original questioner:
This is why I'm trying to avoid the urethane barrier coat. I don't care about the expense, but rather the excessive quantities of the material that I'm forced to buy and the extremely limited shelf life. If I could buy this a gallon at a time, no problem, but I've found it impossible to purchase urethane barrier coat and its converter in less than five gallon buckets, and that's a lifetime supply for me. Conversion varnish I've got, and it stores nearly forever prior to catalyzation.

From contributor R:
If you take the 5 gallon can and break it down into 1 gallon cans, the product has 2 times the shelf life. It is the air in the can that makes the short shelf life.

From contributor J:
You do not have to scuff using the catalyzed urethane prior to sealer, and adhesion is excellent. The one thing I could see as a problem is that your adhesion is only as good as your barrier coat. Urethane has a high build, but low visc, which allows it to penetrate into the wood more than conversion varnish. You can also coat polyester sealer with lacquer, but I don't think you would want to use that as a barrier coat, either!

What was mentioned makes sense about the urea and formaldehyde taking that long to completely get out of the conversion varnish prior to using the polyester. It's one thing to have a coating cure on top of the coats and another to have it cure underneath the coats. I would like to know if you did in fact test your panels for adhesion and what your results were.

From the original questioner:
So far, my conversion varnish as sealer panels look great. It's only been two weeks, though.

To contributor R: I live in Arizona. Sounds like you guys should try out Bloxygen. It puts an inert layer of argon over the material. I use it on oil-based varnishes all the time. Works great.

To contributor J: crosshatch adhesion test shows no issue. I used 15% thinned conversion varnish so that the flow was good. I never used un-thinned C-V, as it always pore bridges if I don't and I hate that. Sanded with 320 prior to the polyester. I would sand prior to the polyester to smooth the surface down anyway, so sanding the barrier coat doesn't bother me.

To the original questioner: We do ship in smaller quantities. In fact, our standard barrier coat is packed in 2.5 gallon pails (10 liters).

I would be very concerned with C-V under polyester. Both for the reasons above (two different catalyst types) and also, there are not too many finishes from lacquer to latex that will not adhere as a topcoat on polyester. Once polyester is cured, the only adhesion you get is what you create with sanding. Underneath polyester is a totally different question. The main solvent in polyester besides the styrene is acetone. The acetone has the potential to re-wet the C-V and open the finish up to adhesion and blushing problems from the migration of the acid catalyst.

I also would be concerned with lifting, as a window has to be present (time unknown) when coating a very hard finish (polyester) over a less hard varnish. Once the urethane is applied, a good rule is to wait 2 hours and apply the polyester without sanding. This saves time ($) and will allow you to fill your substrate all in one day. With a C-V underneath, I would not recommend re-coating without sanding or a good C-V cure.

We have great track records with polyurethane under polyester and not C-V. Most of us are less concerned with your choice of topcoats, but would like you to follow the system for filling as tested.

The thin barrier also helps with silvering and wetting of the substrate.

From contributor J:
You still haven't said if you tested your panels the way I suggested. Polyester adhesion is not like any other coating as far as testing. A crosshatch test may yield favorable results, but until you hit it with something you won't know how good your adhesion is. You may not be putting your polyester sealer on as thick as we do, either, and this may make some difference. You got me curious about how you would do a crosshatch test on a thick coating now. We have tried and it just scratches the top of the polyester and never gets down to the substrate, even with the lacquer top-coated finishes. Hit your test panel with a hammer to see what happens. The polyester finish on our Baldwin Artist Grands gets tested up to 80psi and all it does is leave a dent with cracks around the indention, but the polyester is still tight as a drum.

From contributor M:
Isocyanate urethane is the only way to go. CV is not as strong a coating as the urethane. Ever try and strip a polyester conference table? I have and it's not a pretty picture. Stick to what the manufacturer recommends. Also, urethane adheres very well to oily woods. CV does not. As to the shelf life of the isocyante, just get a small tank of nitrogen and squirt a little in the can before closing. Nitrogen is heavier than air and will stay where you put it. I used to work at a factory where we manufactured Imron copy urethanes for aircraft and that's how we packaged the material. Lasts for years that way.

From the original questioner:
I've got my trusty ball-peen hammer in hand and whack! Where the ball of the ball-peen hammer hit looks like a spider web. Then there's an area of de-adhesion between the topcoat of C-V and the polyester beneath it. Oddly, I can smell the polyester under the area that I am now peeling away with my fingernail. (About a quarter of an inch around the depression left by the ball-peen.) I don't see any problem under the polyester. How long after the polyester is sprayed should you wait before top-coating? I waited overnight and I'm surprised that I can smell the polyester. (A very distinctive odor, like Bondo.)

From contributor J:
You should be ok with top-coating the next day. How fine did you sand the polyester? The only thing I could think of from this is that the polyester was sanded too fine for the conversion varnish.

It is styrene that you are smelling and you will always have that smell when you start to get back into it, cured or not cured. When you sand that stuff again, you still have that styrene smell. Bondo is in the same family as polyester. In fact, it is a form of polyester just like fiber glass resin. They all contain styrene and are cured with MEKP @ 2%.

Since your C/V sealer and polyester fill have a good bond and it's just the topcoat to polyester, you will have to work on your sanding.

One possibility could be that the formaldehyde, acid catalyst, or urea could be migrating through the polyester and causing the topcoat to turn loose. The only way to know for sure now would be use the same process without the C/V as a barrier. The thing about a catalyzed urethane barrier is that you don't have issues with the colorfastness of your dyes or pigment in the stain. Also, it is thin enough to go down into the pore of the wood and helps reduce the trapping of air and bubbling of the polyester. You could do a panel with polyester fill then topcoat less any type of barrier, but it would just be the chance of trapping air that could cause you problems and the colorfastness of your stains. If you decide to try that then don't sand your white wood over a 150 grit, as if you get it too slick it won't stick.

From the original questioner:
I sanded the polyester to P320 prior to using my Sherwin-Williams Water White Conversion Varnish topcoat. I sanded the C-V to P220 prior to putting down the polyester.

From contributor J:
The sanding sounds good to me. You shouldn't have had anything come loose, though. Unless you whacked it pretty good. Our test is mostly controlled. It starts at 20 psi, then in 20 psi increments goes up to 100 psi. Got an old lacquer panel in my hand right now. Poly sealer black lacquer topcoat. Just dents with cracks around it. Did the crosshatch on top of the poly to test the adhesion of the lacquer on top of the polyester. All looks pretty good.

But if you are satisfied with the results of your process and they are projects for your own personal satisfaction, then what you're doing is okay. I hate to put a bunch of work in a finish and then have it fail, because I take a lot of pride in my work and you seem too also.

From the original questioner:
I smashed it pretty good. Got about an 1/8" depression from the ball of the ball-peen hammer.

From contributor T:
I'm getting ready to do polyesters. I thought you could self-seal with this stuff. What is the benefit of using polyurethane as a sealer? Why one topcoat with another product, cv or otherwise? Urethane underneath because it's more flexible, and keeps the ester from cracking with subtle substrate movement, and a topcoat so you don't have to cut and polish the ester? Is this right ? I want to do opaque pigmented plastic like finishes and see that polyester is the only way.

From contributor J:
The urethane is for adhesion. It helps on oily woods and keeps from trapping air bubbles and the polyester can be sprayed directly on top of the urethane without scuff sanding, if done within the window. The purpose of topcoating with other than polyester is using the polyester as a fill then topcoating with an easier-to-work finish for rubbing and stuff. Just depends on the look you want. You are correct on your guesses. For what you want to do, I think you would be in good shape to do a polyester primer, then a urethane topcoat.

From contributor T:
If the urethane is for adhesion, do you mean only on oily woods? You suggested that I would prime with polyester--will this give me adhesion risks? Also, if one wants an other-than-gloss finish, would it still make sense to topcoat with urethane? And is the ease the only benefit of using another product on top or does the flexibility of other products keep the polyester (which I understand is very brittle) from cracking at, say, an MDFglue joint?

From contributor J:
Priming with polyester, you won't need a barrier coat unless you are going over oily woods. Just don't sand the wood too fine--no more than a 150 grit. Polyester itself is very flexible. It's just that you are putting it on so thick that when a joint moves, and it will, the polyester cracks. This does happen to any finish but the thickness of the film is so thin that it is not noticed or bridged over like it would be with polyester.

To my knowledge, there isn't a low sheen polyester. I think it has something to do with the polyester being too thick for a flattening paste to work correctly. And a satin hand rubbed polyester--well, I'm still working on it. But yes, ease of spraying and the working out after it's finished is the other reason to topcoat with something besides polyester. You get a great fill with no shrinkage with polyester when you do it correctly.