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Building Tips and technical articles. => Paint and finishing => Topic started by: Brett Buck on September 04, 2022, 04:33:53 PM
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This is not about finishing airplanes, but there are a lot of talented people here - how in the heck do you get a decent finish with water-based polyurethane? I am pretty careful, I have read the instructions and the many internet videos, but I cannot get it to work for a flip.
The problem seems to be that the water raises the grain, every time, so it's like you are back to square one after every coat. I have tried it over bare wood, that seems hopeless, over various grain-fillers, and over lacquer (the idea being to seal it with the lacquer, which works fine, then use that to prevent the water from getting into the wood). At this point I would be happy to have a tolerable finish, but, even with my low standards, it's just no good.
Brett
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I'll be watching with interest because there is so much of that stuff on the shelf, I don't know what to buy! My son Sean uses polycrylic on his free flight glider wings and he says he just brushes it on, then wipes off excess. When dry , he sands smooth and that is pretty much it. Beyond that I don't have a clue.
Type at you later,
Dan McEntee
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I have used it over silkspan over wood as a base, 2 coats, (FlightLines method) sand between then switch to regular paint. Needs a bit longer dry time once dry to the touch before sanding, kinda like sanding silver.
Best, DennisT
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Brett, These mahogany nose cones were finished as plugs for fiberglass molds. I don't know how much you can tell from the picture but they came out very well and it was fast and easy. I used Minwax Polycrylic, 3 or 4 coats sanded with 400 in between. After last coat, sanded with 600/1000/1500 in order. Then they were rubbed out with Mirror Glaze Fine Cut Cleaner followed by Swirl Remover. I applied and polished several coats of Partall Wax before starting on the molds. Best of all, it didin't smell up the house like the Duratec Surfacing Primer I usually use.
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I have to apologize, I wasn't clear about my issue - this was about conventional woodworking, not model airplanes. At any rate - I gave up. I was making some progress finishing up a test piece for a reversible work bench surface/drafting board , but it was not particularly good and it was taking a lot of work. I reverted to melamine-coated particle board, $19 and ready to go.
I also confirmed that regular wipe-on oil-based polyurethane is much easier, and when it is around 100 degrees and 10% humidity, adequately fast to dry. Part of my issue with the water-based stuff is, I think, drying too fast in these conditions. I can wait until the temp goes down to normal, but the humidity never goes above 25-30% indoors and I am told by experts you need about 50-60% to keep it from drying faster than it can level or penetrate.
So, just to get on with my real project, I have punted for now.
Brett
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... but the humidity never goes above 25-30% indoors and I am told by experts you need about 50-60% to keep it from drying faster than it can level or penetrate...
Just add retarder?
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In answer to a PM on the topic, this is how I use regular oil-based wipe-on polyurethane as a finish on hardwood/furniture/radio cabinets (not airplanes - so far...) :
Start with bare or stripped wood
Sand/fill/color match filler as with any finish
Sand all over with 220
Mix up Timbermate wood filler as described in the instructions as a grain filler - scoop out some, add water a little at a time until it is about the consistency of latex house paint. Brush/squeege over all surfaces, finish with squeege/putty knife at low angle at about a 45 degree angle to the grain and a lot of pressure. This forces it into the pores and grain and removes excess. Do it enough to leave a haze over entire surface, any excess is just wasted effort later. Let dry a few hours or overnight.
Sand with grain with 220-240, remove all haze from filler. The intent it to just leave it in the pores and grain.
Shake up polyurethane can, pour into paper towel or rag, liberally, and just wipe it around everwhere. Don't leave puddles but cover everything well.
Wait at least 2-3 hours, or overnight, until most of smell is gone and the surface is not sticky. This week, at around 100 degrees and 10-15% humidity, I could go at about an hour but it usually is a pretty slow drying material
Sand with the grain, 400 grit enough to smooth it everywhere and leave no shiny or untouched spots. You should get dust and no clogging, if it clogs, wait longer.
repeat, same thing, coat, wait long enough, sand with 400.
I repeat for at least 4 cycles, 6 makes it mirror smooth if desired. Last coat, don't sand it, you are done.
I usually use satin, but if you wait a few days and use gloss, you can rub it out shiny. Gloss usually takes much longer to dry.
Let it sit a few days before putting any pressure or load, but after a few days-week, it is really tough stuff. Good luck ever getting it off later, sandpaper is your best bet, because most chemicals won't touch it.
Brett
p.s. Here are some pictures. This was just some hardware store mahogany plywood, not the CDX construction stuff, but not anything special. The grain filler was tan/maple color. The color is natural with no stain. The surface is completely smooth, there is no grain texture at all, but obviously you can see clearly down to the wood. Ignore the chips, I didn't bother filling and matching them for a test piece.
Also, while this is Minwax Wipe-On Poly, satin, straight from the can, all it really is the the regular oil-based Minwax poly that has been heavily thinned with mineral spirits. You could easily make your own that way, but, I don't use a lot of it and a pint can of the pre-thinned stuff it plenty for my purposes
And no, I will not show the other side, where I tried the water-based type! I am sure it is just a matter of my poor technique, but, "poor" is the operative word. Suffice it to say, the oil-based side is much better.
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It looks like they sold you oak plywood.
I make cabinets and other things out of wood for a living. I've used a lot of finishes over the years.
I used a lot of water-borne products and most of them use the words 'non grain-raising' somewhere on the label.
I have discovered that just like politicians ... they are liars.
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It looks like they sold you oak plywood.
Yes, oak on the back side (this one), the other side is phillipine mahogony. That's the one I am disinclined to show!
I make cabinets and other things out of wood for a living. I've used a lot of finishes over the years.
I used a lot of water-borne products and most of them use the words 'non grain-raising' somewhere on the label.
I have discovered that just like politicians ... they are liars.
So it's not just me! The dilemma appears to be that if you sand it between each coat, you keep exposing the wood which then gets raised by the next coat. If you do maybe 3 coats without sanding to put enough film to not go through, you can never get enough of it off. It's really tough stuff, once it gets good and dry it's at least as hard as automotive clear, almost as hard as the old catalyzed acrylic stuff we used to use, the sandpaper just skids across it, so you have to use a lot of effort - which makes it easy to go through again.
Brett
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My best results after some trial and aggravation.
Whisker it first. --Wipe with a damp rag, let dry, sand again.
One coat - not heavy.
Sand .
Another coat.
Sand again.
Next coat. Cross fingers and hope.
Never, ever put steel wool on it.
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My best results after some trial and aggravation.
Whisker it first. --Wipe with a damp rag, let dry, sand again.
One coat - not heavy.
Sand .
Another coat.
Sand again.
Next coat. Cross fingers and hope.
Never, ever put steel wool on it.
Why- does the next coat rust it? Or does the oil on the wool screw it up? I wasn't inclined to do it, anyway, but I am curious.
I did figure out to raise the grain of the bare wood first, but it kept doing it on every coat afterwards - maybe I didn't do it enough the first time.
Brett
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The oil on the wool is bad but you cannot hope to get all the steel removed and it WILL rust.
Bronze wool works okay.
Raised grain is big problem with them.
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All the cabinetry shops ive worked at use conversion varnish, which is a 2 part lacquer. For all Natural cabinets We spray 1 sealer coat, sand, then 1 topcoat. Simple and bulletproof.
I guess thats also part of the reason i sometimes wonder why we as modelers with our wooden airplanes keep leaning toward the automobile finishing products instead of the cabinetry industry which is specifically formulated for wood.
Kevin
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All the cabinetry shops ive worked at use conversion varnish, which is a 2 part lacquer. For all Natural cabinets We spray 1 sealer coat, sand, then 1 topcoat. Simple and bulletproof.
I guess thats also part of the reason i sometimes wonder why we as modelers with our wooden airplanes keep leaning toward the automobile finishing products instead of the cabinetry industry which is specifically formulated for wood.
Kevin
Not too many people paint their furniture metallic purple.
Brett
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You can get any color of paint you want in the cabinet industry Brett, Including Metallic Purple.