Spar urethane

I recently finished a clock for my brother. I wiped the sawdust off with a rag and mineral spirits. I gave it ample time to dry and sprayed it with spar urethane. It left a lot of white spots or streaks on the wood. (Walnut)
Should I not use mineral spirits to clean the wood before using spar urethane? What do you guys use to wipe off sawdust residue before putting a finish on it? Denatured alcohol, acetone or turpentine??

 

https://www.tacomaworld.com/threads/any-woodworkers.271012/

 

Nothing fancy for me, I just blow stuff off with my air compressor to make sure all the dust is off.

 

I have a compressor but it’s a small one. Only 5-6 gallon. It works ok but I am just looking for other methods. Thanks.

 

water...

light-colored, damp, microfiber towels. 2 or 3 oughta do it. when the cloth comes back clean you’re done. let the workpiece dry completely before finishing.

 

Old thread but seconded on the compressed air blast. I did professional cabinet and furniture work and when I did finishing, I would use my hand to wipe away the large amount of dust ahead of the air blast. I never did any pre-treatment, I usually used conversion varnish. ANY moisture under the spray would cause the white spots/bubbles. That's why dry air is the best.

We had a big shop compressor, but you can get a 150 psi at HD for not too much.

 

Lacquer top coats will turn "milky" when sprayed on a wet, rainy or very humid day. Furniture plants will mix in a "retarder" chemical which slows down the drying process to prevent the white stuff. Spar urethane may do that also. I've got some spar varnish that I've used on a wooden fishing net for water protection but I'm not sure if all this affects urethane. Sometimes stain, filler or glaze coloring coats not dried completely will cause the milky look also...again on wet days.