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Jul 20, 2012, 10:07 PM
springer's Avatar
VT: While you're correct, some of us here in US do in fact use the real wbpu to good effect. (I think it's actually a better, stronger finish than polycrylic). In my case, I bought a gallon of the water based urethane varnish for a hardwood floor redo, and have been using up the leftovers on my planes. So, either will work, but you are correct that the terms are used interchangeably.
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Jul 21, 2012, 04:26 PM
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vtdiy's Avatar
Yup, Springer I have bought here, and use, true WBPU as well. Very different stuff than Polycrylic.But I would call the terms confused, rather than interchangeable.

An example of the problems confusing these terms causes occurred with a tissue decal sticking recipe featured in a great forum thread, which called for WBPU (Minwax Polycrylic, actually) mixed with white glue and glue stick.

Builders in non U.S. countries messed up some decals, and spent good money for water based polyurethanes when what they actually needed was acrylic medium or varnish, available nearby in art supply stores.

If someone really wants to specify a water based polyurethane, that's great. But to use the term for Minwax Polycrylic as so frequently done causes problems.

Early on a number of years ago in a JMorgan thread on finishing, I believe, a true WBPU (a varathane product if I remember correctly) was use initially for laminating fine glass cloth, and then Minwax Polycrilic was found to be better for the purpose, and so the switch was made. I think the WBPU acronym followed that. That may have been the source of the first confusion. The interesting thing is that the acrylic was considered better than real WBPU for the purpose. So, not really interchangeable even here.
Jul 21, 2012, 08:43 PM
springer's Avatar
Interesting history. On foam the only thing I've noticed is the urethane seems a bit harder once cured than polycrylic.
Jul 22, 2012, 12:06 AM
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vtdiy's Avatar
I've built larger structures than model airplanes with true WBPU over foam, laminating with dacron over. Over a year period there was a gradually worsening problem with gas bubbles and delamination of larger areas of cloth. I think that may have been a result of off-gassing under the cloth as the WBPU reacted with exterior moisture and condensation under the skin over time. Waterbased polyurethanes are odd inhibitions of the normal cure cycle since they're dispersions in water. Polyurethanes normally cure by reaction to moisture, and as those who use polyurethane glue know (as do users of Great Stuff foam) off-gas in the process. Acrylics polymerize in a completely different way, and don't offgas at all.

The true WBPU structures also have an amber tint to start with, and that color deepened with exposure to sunlight.

Acrylics (like Minwax Polycrylic) dry water clear and do not seem to yellow with sunlight. I think Minwax Polycrylic should be called WBAF for "water based acrylic finish" to avoid confusion with polyurethanes, and help builders in countries where Polycrylic is unavailble to find an equivalent product.

Those are some of the differences I've noticed besides the problems that occurred when trying to mix true WBPU with white glue/glue stick in the printed decal recipe mentioned earlier.
Last edited by vtdiy; Oct 12, 2012 at 07:55 AM.
Jul 22, 2012, 12:50 AM
Foam is where the heart is
brett.c's Avatar
Originally Posted by NOI53Y
Hey guys,

I'm relatively new to the world of foamies and have some questions about WBPU(Water Based Polyurethane).

I understand people use it to make their foamies stronger but what sort of foam is it suitable to use on. Does it work on EPS, Depron and so on.

Also what is the deal with using matting with it. Is this similar to using fiberglass.

If there is any links to information about using it then that would be great because I did a search and couldn't really find what I was after.
Hi Glenn,
I use Cabots WBPU on Depron, EPO etc.
You can add it straight as a sealer or use it with glass tissue for strength. Or even newsprint.
You can also mix it with talcum powder to make a good filler. And a nice smelling plane
If you sand EPO it comes out all fury like. Apply a wbpu/talc mix and it can be sanded smooth as a baby's. See pic below of an EPO 190 that I highly modified and treated with wbpu/talc.

It can add a fair amount of weight to a plane so you need to take that into consideration.
Oct 12, 2012, 12:00 AM
*GRAVITY* I hate it :(
Sparken57's Avatar

keep it simple

I think the main thing to remember here is ANY liquid applied to foam should be WATER BASED I guess hot glue & CA could be an exception I dont know what there base is.
Oct 12, 2012, 07:59 AM
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vtdiy's Avatar

Keep it accurate

Originally Posted by Sparken57
I think the main thing to remember here is ANY liquid applied to foam should be WATER BASED I guess hot glue & CA could be an exception I dont know what there base is.
Epoxy isn't water based. Rubber cement isn't water based. POR isn't water based. Testor's spray paints aren't water based, etc.

Polycrylic is acrylic, not polyurethane.

Water based polyurethanes, are polyurethanes.
Dec 02, 2019, 09:07 AM
Registered User
Not disputing anything you've said, but System 3 does advertise a water based two-part epoxy primer I intend to try.

In another post, someone said that the cross linker sold by System 3 works well to fuel-proof Minwax Polycrylic, so good job pointing out that WBPU is not the same thing!

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