Covering with WBPU

This article gives advice, hints and tips from an experienced club modeller. Please note that the SRFC accepts no liability or responsibility for any loss or damage resulting from actions taken by anyone acting on advice given in these articles. If you have any questions or would like to contact the authors please contact us

In the beginning

Some while ago I penned an article on covering with glass cloth & epoxy resin. I’m pleased to say that it had the hoped-for effect of launching other members on their own voyage of discovery with this covering medium. It doesn’t suit everybody, however, as it can become a very sticky business, and rubbing down the cured epoxy can turn into an epic!

Technology moves on, and modellers ingenuity follows – with the popularity of wooden floors on the increase, some interesting varnish finishes are appearing on the market – the one that is the subject of this missive being Water-Based Polyurethane (WPBU).

It has been designed to provide a hardwearing, shiny finish to wooden floors. Applied by brush or roller, subsequent coats can be applied after about 1 hour without the necessity of sanding between coats. However when the varnish cures, it is as hard as any epoxy type of finish (well, that’s what it says on the tin!)

It seems to me that WPBU has all of the advantages of an epoxy resin, but without any disadvantage –

  • It’s low odour
  • It brushes on easily
  • It’s recoatable in 30 minutes
  • It sands easily
  • Tools (and you!) clean-up in soapy water.
The only real problem I have come across is that you require many coats (8 to 10!) to fill the weave of the glass cloth. As you can apply 2 or even 3 coats in an evening, it isn’t too big a deal – but I have a cunning plan…

Covering a model is similar to any other covering medium (tissue & dope, Nylon & dope, Solarfilm, Solartex, etc) pieces of glass cloth are cut to size and then stuck to the model using a brushed on coat of WBPU. Pull wrinkles out as best you can, snipping the covering to negotiate humps, bumps & curves. Once each piece has dried (about 30 minutes in a warm room) you can trim the edges and cover the opposite side.

Once the entire model has been covered and has dried, a quick clean up with fine sandpaper should result in a smooth, matt surface.

Glen asked me for an article after I’d got to this stage of covering a GWS Formosa, so pictures will go from here.

I used half-ounce per square yard glass cloth to cover the model, a moulded foam electric model. I could leave the surface uncovered, but being moulded from a soft white foam, it gets tatty so quickly.

So, first piccie shows the model filled, covered and sanded, ready for subsequent coats of WBPU
Formosa 3
Picture two shows the lightness of the cloth
Cloth
Another coating of WBPU ensures a good solid base, with the glass cloth well stuck down, and a sheen starting to develop.

The eagle-eyed of you will have noticed that the control surfaces are still attached to their parent bits. This is done to prevent the possibility of distortion when covering separately. Before I go on to the next stage I shall cut the surfaces off, and finish the exposed edges with WBPU.

My cunning plan!
I could just continue adding more coats of WBPU until I have filled the weave completely – I could leave it as it is and allow the weave to show though a little. But by taking advantage of another modern D.I.Y. miracle, I can fill the weave in a further two coats.

Picture three shows ‘the usual suspects’
WBPU
Take a blob of that other miracle substance – ready-mixed lightweight filler – and plop it into a bowl. I use Polycell ‘no sanding’ filler; check for the little circle on the tub that states, "Light weight filler". Stir and slowly add WBPU until the mix resembles melted ice cream. Brush on two coats, allowing about 10 minutes between them, of this slurry all over the model and then let dry for 24 hours. Now sand to a perfect finish – as the mix is waterproof, you can wet sand without the filler going mushy.

The model now has a nice hard skin to it; ready to take paint, stickers and hanger abuse!

This Formosa has been re-motored with an E-Flite 370 (5400Kv) Brushless Motor, as per the ‘how to’ on the SMC SMC Website.

Electricity rules!

Tom Gaskin

Last updated on 13-March-07