How to get less-lumpy polyurethane finish?

[earthtonesaudio]

I just sprayed on a second coat of oil-based polyurethane (this kind) and it looks rather lumpy. Is this because the poly is starting to cure, or am I applying it too thick, or did I not wait long enough between coats, or some combination...? Tips welcome.

[vigilante397]

I had quite a time getting this right, but a million REALLY THIN coats seems like the way to do it. It takes forever, but it gives a smooth finish. I've also found that baking in between coats helps it smooth and harden.

I've also found that some particular cans (or maybe just the spray tips?) are better than others. I have a red that comes out perfectly smooth every single time, and I have a can of white that lumps no matter what. With the white I always end up just doing a lot of clearcoat and sanding it smooth as best I can.

Hope this helps at least a little, and sorry if it doesn't at all

[earthtonesaudio]

That is helpful, and confirms my intuition as well as the opinions given by a couple coworkers I asked. Thanks!

[GGBB]

That looks like "orange peel."  From what I've gathered, it happens if the poly starts to set before it has leveled out fully.  This has multiple causes - paint not properly mixed; doesn't spray evenly; too cold, too warm, too humid, too dry.  Too thin a layer can cause this because it dries faster than a thick layer.  Some good insights here: http://www.avsforum.com/t/1177246/eliminating-orange-peel-finishes#post_17133013.  This guy says a thick layer (but not too thick) is the best approach with spray cans.  I've gone the thin layer route and it just makes the effect a bit smaller.  With a satin finish, I kind of like how it looks and feels when it is small (only noticeable up close).

[earthtonesaudio]

I'm actually using one of these spray guns.
I've been spraying under a high-flow fume hood in a cool, dry shop.

A couple other things that might have contributed to this orange peel effect might be: the pigment had started to settle, the poly might be thickening up inside the spray nozzle, etc.
This is also the third coat, the piece next to it has just two coats and doesn't have the orange peel effect.

I guess the moral of the story is I have too many variables going on at once. 

[tranceracer]

You also can wet sand the finish w/ 2000 grit and clear coat 3-4 more times, wet sanding between each coat.

[PRR]

> spraying under a high-flow fume hood

I don't think household handy-person spray cans are blended for a stiff wind.

I don't think even car spray is intended for huge wrong-way air flow. (There's a flow, but nothing like some fume hoods.)

You could be pulling solvent or solids up the hood instead of getting it on the panel.

Since it's just a few square inches, turn hood, off, spray, then turn hood on.

[R.G.]

Painting is a tech specialty, just like electronic design is.

When you spray paint, the paint liquid is broken up into tiny droplets by the force and turbulence of the gas moving it out of the nozzle. When these droplets hit a surface, they stick and spread out to some degree. When the surface gets sufficiently coated with droplets, they flow together. How much they then flow level depends on the interplay of the liquid viscosity, the surface tension of the liquid, gravity, and how fast the stuff dries. Some lacquers dry so fast that some of the droplets are dry by the time they get from the nozzle to the surface and they either bounce off or stick to earlier droplets that hit.

That being the case, only paints with low viscosity and high surface tension as well as medium drying time can make a really level surface.

When a painter wants a truly level, smooth, high gloss surface, they either turn to truly high tech paints, or go to hand rubbing. Hand rubbing is the process of painting, abrading the surface smoother than the paint went on, spraying again, abrading again, etc. The final abrasion process is with superfine abrasives and polishing compounds. This is the only way to get a truly high gloss and leveled surface.  Think of the top of the Steinway grand piano for inspiration. Those are (or were) hand-rubbed.

Many coats of moderately retarded (that's a chemical additive... ) lacquer with fine block sanding between every few coats can do an OK job.

The real question here is "how much is smooth enough?" The simple answer is to change paints to fit your existing process, or change the variables in the process until you get what you want. A little research on a forum entertaining custom car finishing would not be amiss, either.

[GGBB]

Many coats of moderately retarded (that's a chemical additive... ) lacquer with fine block sanding between every few coats can do an OK job.

If you are going the light coat route, be sure of the "between every few coats" part - you need to lay down enough poly so that some is left behind after sanding.  Also be aware that multiple thin coats could make the surface actually bumpier than a single thicker coat (in my limited experience) requiring more sanding (and more material removal) to work it back to even.  For at least the first coat or two, you might want to try using a foam brush to apply a thicker coat than spraying would, which avoids the air travel issues that affect leveling and drying and can be sanded after each coat.

[earthtonesaudio]

Thanks for the advice, everyone.
I employed almost all of it and varied a few other parameters over the course of several more trials. Now I'm quite happy with the results:


No one asked, but this is how I'm ultimately measuring success:

[Quackzed]

i think that generally, with store bought spray cans, you want to do 3 or 4 thin coats, that look matte and let em fully cure in between, then you have enough texture on the surface for a nice wet coat to stick to without running, the more texture the less likely the wet coat will run, or rather the more leeway you have between the paint flowing on nice and evenly and getting runs. but its a tricky thing to get just right... too much paint/ too close/ too slow moving et al. WILL run at some point the trick is getting the wet coat thick enough to flow together and look wet and stopping before its too much at once and starts to run.
oops, well this post is for posterity, looks like you got a good result already!