Spraying Polyurethane...

[Michael Schumacher]

Over the weekend I went to Lowes to get some window trim - proceeded to stain it Saturday. Since I had a nice spray gun, I decided to spray on the varnish. I had some varnish that was used last time I did a project and it might have gotten thicker over time, but it did work, but I'm wondering if I could thin the next batch with some mineral spirits? After I was done, I did thin it down of what was leftover, about 50/50 mix and it went through the gun a lot better.

So - would that be okay to do?

Michael

[Jim Becker]

And oil based product can be thinned with mineral spirits (paint thinner) to help you spray it. 50/50 is typical of a wiping varnish and is safe if that is what you need to do to get it to work with your gun. (Water borne finishes should not be thinned or no more than 10% if you cannot avoid it) But personally, I don't spray oil based products because they dry so slowly that any overspray is a messy nightmare...

Now as to using old finish...be careful. It's better to buy fresh finish in a quantity appropriate for "today's job" than to risk old varnish not curing and yes, another messy nightmare. Oil based finishes begin to cure when you open the can and many do so at a rate that makes the shelf life relatively short.

[John Mihich]

I thin with napha. Works well and evaps a lot faster.

[steve reeves]

I thin with napha. Works well and evaps a lot faster.

Same here. I get much better results than using mineral spirits and the drying time is a lot less.

However, I use water based finishes whenever possible as the clean up is much quicker and as Jim said... overspray isn't nearly the mess.

Just a few years ago I wouldn't have dreamed of using water based anything... but those products have improved dramatically in recent years. I still prefer oil based stains though... odd.