I'm having trouble with polyurethane (Helmsman semi-gloss) drying much glossier than it should – like an extra-gloss instead of a semi-gloss. I'm using it on stained wood moulding, and when I did the majority of it in the winter, I got the expected semi-gloss sheen. But now It's coming out extremely glossy and does not match the rest (which is already installed). It's the exact same product (although a new can) and I even tried buying another can in case the can was defective, but I'm getting the same result. The only thing that seems to be different is the weather and possibly the conditions (heat) in which the cans were stored. Can exposure to heat in the can or while drying affect the sheen of polyurethane? Is there anything I can do to compensate?
Some pics:
Best Answer
The gloss in clear finishes is reduced by flatting agents which are suspended in the clear finish. Flatting agents are tiny particles that reflect some of the light back, thereby reducing the sheen of the finish. If they settle out or are not mixed well, then the finish will be more glossy, so it's possible you did not mix the finish as well the second time.
Finishers often start with full gloss for the first coats, and then finish with the lower-gloss levels desired, so you may be able to fix the problem simply by applying properly mixed finish over the existing finish.
Another way to reduce the gloss level is to essentially scuffing the surface with fine scratches. This is sometimes called 'rubbing out' the finish. You can do it using various powdered abrasives, fine sandpaper, or scotch-brite pads and sanding with the grain of the wood. The advantage to the mechanical scuffing is that it gives you complete control over the degree you reduce the gloss. However this treatment can be much more time consuming and this would be more typical for fine furniture rather than for trim.
You can read more on adjusting sheen of a finish in this article written by Bob Flexner, a very well known wood finishing expert.