Epoxy can get kind of thick in the bottles if it has been sitting a long time or in the winter. I put the bottles in the microwave oven for about 20 sec. or so and they get right back to normal. Been doing this for years with only good results. I have been doing some building in the garage this winter with temps in the 50's. Usually after applying the epoxy I'll take the parts into the house to harden just for peace of mind. The left over mix seems to harden hard as a rock in the garage though, but does take a little longer to harden. This is with 30 minute (or less) epoxy. When I used to vacuum bag wing skins using West System 24 hour epoxy, there was a strong belief with the sailplane pilots that if the room temperature was not up to specs or close enough, that this 24 hour epoxy would NEVER really cure. I didn't want to find out. When mixing micro balloons or milled glass when epoxy, I know you want to thoroughly mix the epoxy before adding the filler. Yes, clear polyurethane works great for painting the tank and firewall area with. Might even be easier than mixing and thinning epoxy.