Covering with Tissue and WBPU
I like foam for many reasons but to me it looks cheap and nasty and spoils the look of a model. Although it is very easy to build a foam kit, the fact is that foam is damaged very easily. If you have built a foam kit you will know how easy it is to mark the surface of your model before you even get to fly it.
So what can we do to protect and give a nice finish to our foam models? One of the answers is to cover them with tissue and WBPU, or Water Based Polyurethane. So what on earth is WBPU? The stuff I use is a floor sealer and it is very easy and quite safe to use compared to some of the epoxy products. I use my fingers to smooth the tissue during application and when it's dry it peels off them like a layer of skin which is kind of fun
I can't take credit for this method of covering and I would like everyone to know that my inspiration to use the method came from Matt Haltons build thread for his Ultrafly P-51D Mustang here
His model is a great example of what can be done with a foamy.
This is what you will need for the job:
1 can of WBPU. A 1 litre can is enough to cover 4 to 5 small to medium size models.
Enough Silkspan (or Japanese tissue. Google it if you want to know more about it) to cover your model. Silkspan comes in different weights. I use the thickest I can get.
A paint brush about 2.5 cm wide.
Around 120 and 320 grit sandpaper.
Lightweight spac filler. I use NHP Micro-Fill Super Light Model Filler available at hobby shops.
A pair of sharp scissors.
So when I get a new foam kit here is what I do.
First I sand all the bumps and raised bits of foam lightly with 120 grit sandpaper until the surface is smooth. I then fill any low areas with the filler and sand it smooth when dry. When you are happy with the surface preparation it is time to start covering.
Cut a piece of Silkspan big enough to cover and overlap the piece you are about to cover. Make it too big so you can trim it back once you start to apply the WBPU.
Wet your brush and start brushing the WBPU on the Silkspan so it soaks through and sticks the Silkspan to the foam. It is possible to remove the tissue from the model if you need to until the tissue is nearly dry. Start from the middle and don't wet the edges yet. Trim around the model part so the Silkspan will overlap when you cover the other side. If you get the Silkspan wet it is almost impossible to cut. When covering compound curves you will need to cut or tear the tissue so it will conform to the surface of the model. You will find that sometimes it is easier to use small patches of tissue than large ones, but I always try to use the largest pieces I can manage. Typically I can do the bottom of a one-piece wing with one piece of tissue and the top with two. The fuselage can usually be done with two pieces, one for each side. If it is too hard use smaller pieces of tissue. You will find that the tissue will stretch a bit once wet and that you can smooth out most of the wrinkles and bubbles with your fingers. If the tissue doesn't conform to panel lines etc, run your finger or fingernail along the lines so they show through the tissue.
When all the covered pieces are dry, sand them lightly with the 120 grit sandpaper until it feels smooth. Concentrate on smoothing down any wrinkles, areas where the tissue overlaps and areas like wingtips where you had to cut or tear the tissue to make it conform. This is usually where the tissue overlaps so you can sand it down until the cut edges of the tissue disappear. Be careful not to sand through the tissue.
Once you are satisfied that you have sanded all the parts so there are no edges of tissue visible and that the surfaces are nice and smooth, brush a very thin coat of WBPU onto the pieces. It will dry very quickly this time. Then sand with the 320 grit sandpaper and then brush another very thin coat of WBPU. Sand and coat until you have a nice smooth surface.