Need some help on a spray finish

[Eurekan]

I'm building a crib for my next grandkid. Typical combination of solid oak wood, oak plywood and slats. There are so many slats I decided to spray a quick-drying oil polyurethane. I'm not an expert sprayer, only done a couple of small pieces. I'm outside at around 80-85. In the shade on a calm day. When I started I had the feed set too high and it started to run. Wiped off, backed off and went on.

3 coats later looks good. No runs. No sanding between coats. The solid wood and the slats are sort of OK. Rougher then I want. The plywood feels like sand paper. 400 grit knocks it down without visible scratches. I thought it was dust but the 2 surfaces are so different.

I have 2 more pieces to do and I want to avoid this problem. What can I do? Am I spraying with not enough varnish? I can't figure out why the plywood is so very rough. Ideas?

[Kansas City Fireslayer]

It's hard to say why you're getting such a rough surface for sure. It will help if you let the forum know what kind of sprayer you're using.

One thing that comes to mind is the dust you're going to pick up being outside using an oil based poly....even a fast drying one.

Are you sanding your plywood with at least 220 before starting?

If you think you're getting enough material on the piece--and it's still rough--you could consider rubbing it out with 0000 steel wool and a paste wax after it cures out more. That will knock down any dust nibs and give you a nice satin sheen.

If you're sanding in-between coats, make sure you're riding the piece of all the dust prior to adding another coat. A shop vac with a brush will typically do it. You could also use a tack cloth or rag lightly dampened with mineral spirits. You may also consider doing a "wipe on" finish for your last coat and bring it into your garage if you feel safe doing that. There should be less dust in your garage than being outside. Just a few ideas.

[Eurekan]

Thanks for the feedback. Using an HVLP gravity feed gun. Sanded everything to 150 before staining.

[Kansas City Fireslayer]

Did you by chance use an oil based stain? A water based stain would raise the grain which could explain the roughness.

[Eurekan]

Oil based stain. Goodv thought though.

[jteneyck]

It sounds like you either are not spraying a heavy enough coat or you are getting overspray on the finish. Normally, neither should be a problem with an OB varnish, but if the the humidity is really low then maybe it's setting up before it can flow out. The other possibility that just came to mind is that you don't have enough finish on the wood yet. Can you see a real film on the wood? If you can't then there just isn't enough finish built up yet to overcome the underlying roughness. Hope that helps.

John

[Chris S]

You need to sand between coats. Oil poly will raise the grain, and subsequent coats will not make this go away. You have to cut through this with sandpaper and level the finish.

The other important reason to sand between coats is for proper adhesion so the coat you are spraying locks in with the dry coat.

[Herebrooks]

I think your problem is with the first coat. I apply a coat of sanding sealer(oil base) with a brush or pad and wipe it off immediately. That raises the grain and freezes the fibres so you can sand them with 320 nofill paper. That sets you up for your finish coats. With most HVLPs you only have control over viscosity. I use naptha as it's less oily and dries quicker than mineral spirits. You don't need thick coats but the finish needs to be able to run together. Spray with the piece horizontally when you can. Obviously, too thin and it will run. Scuff sand between coats.

[Papa Jim]

I apply poly out of the can with a good foam brush sanding  between coats with 220. After 2 or 3 coats (when it's beginning to look good) I switch to 320 and finish it with a rattle can of poly.  (My substitute for spraying system)