Poly Finishes

[djenkins]

I am building an "Ultimate Miter Saw Cabinet" for my new Bosch SCMS and have some finish questions. First, the carcass is made from 3/4" Cabinet Birch with one side finished. (I found this cabinet grade ply on sale here in town. Cheaper than the good birch ply at HD.) The finished side looks like oil-based polyurethane as some small strips at the ends had yellowed where the sheet had stuck out from under the stack for a while.

My questions are:

1. I have just purchased the HVLP sprayer from Rockler and want to know can water-based polyacrylics be sprayed through and generally how much water do I have to use to thin down for spraying.

2. If I use a water-based polyacrylic to spray the whole project will the finished side still turn yellow or darken under the polyacrylic?

3. If the finished side will yellow I assume it is better to complete the project with oil-based poly? Is the 50/50 ratio of polyurethane and mineral spirits/naptha a good spray ratio?

As always I appreciate all your comments in guiding me in the right direction.

[mschrank]

I've been using the HF version of that sprayer almost every day for the past 2 weeks. I've only sprayed water-base laquer (Resisthane) and not poly, so my replies below are more gut reactions than experience. I've heard that spraying poly can be unpleasant, as the overspray (though little with an HVLP) is so sticky and messy.

My questions are:

1. I have just purchased the HVLP sprayer from Rockler and want to know can water-based polyacrylics be sprayed through and generally how much water do I have to use to thin down for spraying.

Your sprayer should have come with a viscosity cup...the directions say to thin so that it takes 8-20 seconds for the finish to flow through the cup. Water based finishes generally require less thinning than oil base.

2. If I use a water-based polyacrylic to spray the whole project will the finished side still turn yellow or darken under the polyacrylic?
I suppose it depends on if the water-based product contains any UV blockers...it probably does not so the oil base might continue to yellow.

3. If the finished side will yellow I assume it is better to complete the project with oil-based poly? Is the 50/50 ratio of polyurethane and mineral spirits/naptha a good spray ratio?
If it were me, I would stick to the oil base for fear of the water-base having adhesion problems. Poly doesn't play nice with other finishes, so I would be inclined to not try applying water-base over oil-base. As far as thinning ratio, I would again recommend using the viscosity cup.

As always I appreciate all your comments in guiding me in the right direction.

[Tom Slick]

poly can be sprayed but because the drying time is so long it is generally not recommended. the overspray doesn't dry quickly and sticks to everything. laquer, on the other hand, dries extremely fast and its overspray turns to dust immediatly.

[cabinetman]

You can spray waterbased polyurethane. Using for example, Minwax Polycrylic, I thin about 10% with water. It dries to the touch in about 20 Minutes, and does not yellow.

Oil base polyurethane will take on an amber effect with any build and could take hours to dry to the touch, especially if sprayed and thinned with naptha. Depending on the orifice of your gun, the thinning ratio for oil base poly could vary from 20% on up. You would have to try a sample. It's heavy enough to start running when wet. Many thin coats work well.

Lacquer is definitely faster, and because of it's high VOC's is not sold in some areas. I used to spray only lacquer. Because it's so toxic and unpredictable with changes in humidity, I rarely use it anymore. I've gotten such good results with waterbased polyurethane, that I'm using it primarily.

[JimD]

I have sprayed Minwax Polycrylic. You cannot add much water. I use a Wagner conversion gun on a very small compressor. Resisthane sprays a lot better and is a lot cheaper. It is a water based pre-catalyzed lacquer. You can spray another coat in an hour. Nice stuff.

If I wanted the unfinished side to look the same as the already finished side of your plywood, I would probably skip spraying and use wipe on oil based poly. It is almost as fast as spraying and even easier to apply. No cleanup -let the rag dry then throw it away. I buy full strength oil based and thin it about 1/3 with mineral spirits.

You can brush resisthane on small projects but it drys too fast to do this on bigger projects - so you pull out the spray gun.

Jim

[djenkins]

I have sprayed Minwax Polycrylic. You cannot add much water. I use a Wagner conversion gun on a very small compressor. Resisthane sprays a lot better and is a lot cheaper. It is a water based pre-catalyzed lacquer. You can spray another coat in an hour. Nice stuff.

If I wanted the unfinished side to look the same as the already finished side of your plywood, I would probably skip spraying and use wipe on oil based poly. It is almost as fast as spraying and even easier to apply. No cleanup -let the rag dry then throw it away. I buy full strength oil based and thin it about 1/3 with mineral spirits.

You can brush resisthane on small projects but it drys too fast to do this on bigger projects - so you pull out the spray gun.

Jim

Everyone, thanks for the help.

JimD, I agree with you. Due to the fact that I have one finished side I will take your advice and probably wipe a GF Seal-A-Cell base on the unfinished wood, then sand the finished side first coat and follow up with two wipes of GF Arm-R-Seal for the top coats.

JimD, where do you get Resisthane I haven't seen it, but then I haven't looked for it specifically.

I'll just have to wait until I have a new 100% unfinished project before I use my new sprayer and lacquer or Resisthane! I see a new project in the future, hmmm, maybe a new assembly table. Yeah, that's the ticket!

Thanks, this site is just truely amazing!

[JimD]

Hydrocote Resisthane is available from "hoodfinishing.com" for $22.95/gallon - has gone up a little since I bought my first gallon. The ability to clean up with water is a big plus for me. Resisthane is also a good sealer so it is all you need.

Jim