Thread: A few polyurethane Q's

I just started experimenting with polyurethane as a finish. Most to use as a finisher for my epoxy finished scales after I've sanded out any imperfections. I'm quite liking poly too, seems to work great.

I was wondering what the usual FULL curing time for poly is? The instructions on the tin of poly I bought says a day, but I'm just not sure. It says it will cure to a "diamond hard" finish, an obvious over exaggeration, but so far it's been about two days of curing and it hasn't hardened as much as CA or epoxy does. The finish right now is pretty hard, but it can still be marred if I apply pressure with a finger nail. So I was just wondering if this is as hard as it gets, or it just needs many days of curing to become rock hard.

I was also just wondering what everyone else uses to polish/buff poly to a high gloss finish. I seem to always get a few flaws in the finish, no matter what finish it is, and with this poly, I just tried sanding out with #0000 steel wool and then polishing with simichrome. Works great, the finish product is very very near that glass-like look.

Some finishes need lots of dry air. I'm not sure about polyurethane.

I use a wipe-on poly and I usually give it 3 or 4 days to really set up (I don't really do a lot of work on scales other than on the weekend, so I just leave it on Sunday and come back to it Friday evening). I think it also really depends on how thickly you put it on.

I polish my scales with a little brown tripoli on a buffing wheel followed by some Mass.

Mark

Polyurethane is a polymerizing (spelling?) oil. It cures in air.
Several things I know of will inhibit curing:
-High humidity
-Contaminants from the material (oils from wood, leftover solvents, stains that have not dried fully)
-Cold Temperatures
-Thick Coats

Some tips:
-Dilute your poly for the first and second coat for better absorbtion. Mix to about 1:2 thinner to poly. This is especially important for oily woods as it creates a barrier for future coats to sit on that won't allow the oil to leach in. It also will cure faster. Failing this, CA makes a good base coat.
-Never mix or shake poly, this can create air bubbles that can destroy a nice finish.
-Poly has a shelf life. Buy fresh and in the quantities you need.
-Always let the previous coat cure for a day before applying more. In my experience, this makes for less total curing time.

That 1-day cure is horsesh*t for brush-on poly. Poly will continue to cure for several days depending on conditions. It's ready for another coat after a day. Sanding and polishing should be done after it has had a chance to really cure. For best results with gloss poly, wet sand then polish with fine rubbing compound and finish with wax.

A few days to fully gas off, imho.

The time frame also depends on how thick the coating is.

I've found the 'more layers of a thin coat' mentality to contain the best results with regards to finish dry time and color layers.

There are several classes of varnish generally referred to as short oil, medium and long oil varnishes. The long oil types are typically called Spar Varnish and are the softest when fully cured to allow them to be used outside where temperature differences would cause the medium and short oil types to crack and flake and Spar contains UV protectors as well. So, most furniture/flooring varnishes will be medium and short oil types. The curing time (for a complete cure) can be days to weeks for these products even if they are they are sufficiently hard to sand and re-coat in 6 to 8 hrs. Our floors smelled of curing oil for almost a month before it went away... this was not the solvent smell, just the smell of curing oil. I think I'd prefer to just sand and polish the epoxy, rather than add poly over top, but if I were to use poly, I'd use it from the start.


Christian

Originally Posted by kaptain_zero

I think I'd prefer to just sand and polish the epoxy, rather than add poly over top, but if I were to use poly, I'd use it from the start.

I'd love to just do that as well, but I can't seem to polish epoxy to high gloss. Everytime I try to, it comes out to a satin finish instead of high gloss.

Originally Posted by ToxIk

I'd love to just do that as well, but I can't seem to polish epoxy to high gloss. Everytime I try to, it comes out to a satin finish instead of high gloss.

How are you polishing?

What I do:
600-1000-2000 grit WET sanding (as in with water)
MAAS equivalent (I use Rolite products)
Wax

Wet sanding is important. I find the heat of friction from dry sanding smears the surface and causes ill effects.

Like Dan said.... only I'd cheat and use a loose muslin wheel on the buffer for polishing. I've got some woodturning wax/abrasive mix sticks that work a treat that way. Alternately, and just as good is a good plastic polish with the muslin wheel on a bench grinder/buffer.

Christian