"BIG" mistake on interior finish

[bohemian]

This is a plea to all that partake in the finishing/refinishing of wood.
Okay,To start I thought I would start applying the clear finish to the interior of the trailer I'm working on, the finish is applied to some really nice stripe mahogany ply.
This time I thought I would use a finish that was a little more durable than the usual shellac I would apply to a trailers interior. ( I was concerned about condensation)
Now this is where I went wrong, I went ahead and applied Minwax spar "urethane". I actually didn't realize that it wasn't a varnish until after I applied it

So the problem I'm having with this stuff other than the smell, is that I can't lay it out with out having it turn out blotchy.
My technique is the same as I used on my teardrops exterior (Rolled on with a foam roller and back brushed) the finish was applied to raw wood with no stain, in a heated shop, sanding between each coat, allowed to dry days instead of hours between coats. I tried thinning it 10% to 50% with no luck, I'm still getting the same results

So now I'm giving up on the Minwax spar urethane after four coats, and I would like to know, with the help of the group, what product I can apply over the spar urethane that's going to work form me without having bonding problems.
Now I did send off four emails to Minwax asking for their help, but they keep telling me that I didn't let the stain dry long enough( I didn't use stain) Then they said that it's a moisture problem ( woods in a dry heated shop?)
I'm sure there will be a lots of different answers to this problem, and I'm willing to hear them all. Even if it means I'll have to wipe zebra urine on the walls to get it to work. So don't be shy, I could really use your help on this.
Thanks
Wayne

[eamarquardt]

When in doubt, call the manufacturer.

http://www.minwax.com/contact

Hope this helps.

Cheers,

Gus

[VijayGupta]

> I actually didn't realize that it wasn't a varnish until after I applied it

Well, it is a varnish. It's an odd choice (more about that later), but spar varnishes are considered "long oil" varnishes. This means that it has a higher proportion of oil (linseed oil in this case) to resin (urethane in this case) than "short oil" varnish. Short oil varnishes are typically what we use for interior applications. Long oil varnishes have a bit more of the characteristics of oil than resin. This means they are a bit softer and more flexible, and less water resistant. The original design of spar varnish is to allow wood members to flex without cracking.

The reason that it's an odd choice is that urethane resin has a lower UV resistance than other resins commonly used for exterior applications. So while you think it's the cat's meow for external wood, we have it less water-proof and less UV resistant -- a couple of desirable attributes, IMO.

Varnishes cure by polymerization -- meaning oxygen is absorbed and the finish molecules cross-link. This happens after the dilutents (mineral spirits, usually), also known as the carrier, evaporates away, known as flash-off. Now if there is a lot of finish in a small area, the oxygen starts to become is short supply, slowing this process. Ambient temperature also slows, or in some cases, halts the process. The very least I would do is to put up a fan to ensure a fresh air supply while the varnish cures. It would also help if you keep the heat to room temperature. Humidity also has a minor effect. Sometimes people quote the 70-70 rule -- 70 degrees F and 70% humidity.

I am not sure what you mean by "blotchy" This tends to indicate uneven dispersal of the flattener, silica, in the mix. In sheen levels other than gloss, it's important to stir the finish well prior to and during the application. If you add thinner, as you say up to 50%, then the flattener will fall out of suspension even faster because the mix is less viscous.

When I apply varnish, it put on as little as possible to cover completely. Too thick varnish will cure on the surface and form a skin that prevents or inhibits curing lower. If you've ever seen a heavy drip of varnish on a surface, you will note that it's still rubbery inside years later. Thick coats also tend to produce brush marks and plastic-looking finish. The manufacturer's product data sheet will normally tell you the recommended wet mil thickness and it's not much, usually 4-6 mil (0.004" to 0.006")

Assuming you have a good batch of stuff, there's no reason why it should not work OK. If the can was unopened when you started, it should be good. If it's a can left over from a few years ago, then it may have gone bad by partially curing in the can because of the oxygen in the headspace.

[Festus]

So the problem I'm having with this stuff other than the smell, is that I can't lay it out with out having it turn out blotchy.
My technique is the same as I used on my teardrops exterior (Rolled on with a foam roller and back brushed) the finish was applied to raw wood with no stain, in a heated shop, sanding between each coat, allowed to dry days instead of hours between coats. I tried thinning it 10% to 50% with no luck, I'm still getting the same results

So now I'm giving up on the Minwax spar urethane after four coats, and I would like to know, with the help of the group, what product I can apply over the spar urethane that's going to work form me without having bonding problems.

First, could you define blotchy? Do you mean uneven sheen after the finish is dry? Or is the finish blushing which is areas of the finish that have a milky white look?
What area do you live in? Humidity can cause a lot of problems when finishing and a heated shop doesn't neccesarily mean a low humidity shop.
Are you finishing the panels before they are installed or are they already installed and you are sitting inside the teardrop applying the finish?

The striped mahogany look is caused by the grain of the wood. To get an even finish, the wood has to be well sealed. You could have used a paste wood filler to seal the open pores of the wood prior to finishing, but it is rather a messy job. Since you have already applied finish, probably your best bet would be to keep applying more coats and sanding in between, both to create "tooth" for the next coat to adhere to, and to "level off" the high spots. Apply the finish without diluting it if possible. With a urethane or polyurethane you can thin the first coat 25-50% as a seal coat, then apply the next coats without dilution.

If you have ever seen one of the old mahogany boats with a finish that looks like glass, they are usually done with a paste wood filler, sealed, and numerous coats of finish until it levels out like glass. One of my customers restored the wood boats for a living, and he always heated the can of finish in hot water, (hot, NOT boiling) and sprayed it on. By heating it in the water, it would reduce the viscosity and make it spray better and level out faster, but by not adding any thinner, it would also build faster.

If the finish is blushing, it is a humidity problem, and is more of an issue.

Lynn