How to make sanding sealer

When I was a kid, I was really into designing and building model rockets.. as well as chemistry. If one wants a very smooth finish on balsa model rocket fins, you started with the cut balsa, and then, before applying any paint, you first sealed the fins with several coats of "sanding sealer", sanding between each coat. With this done, the paint (usually stuff called "hot fuel proof butyrate dope") flowed on smoothly, often in as little as one coat.

The problem is - I was willing to pay my hard earned schoolboy allowance to buy rocket parts and paint, but.. paying the same price for clear sanding sealer that you had to use lots of... well, I wanted a better way.

After experimenting with a variety of solvents and solutes (yes, some that no 7th grader should have had...), I remember finding and settling on a combination of "lacquer thinner" and styrofoam chips that worked as well as the stuff I found in the store.

Now, years later, I find myself making some masthead flys, and am still as cheap as I was as a kid. My lab is not as well outfitted as it was then (no ether, can you believe it??), but.. the lacquer thinner of modern days does not work. However, I did find a combination that does work, presented below for any other cheapskates on the web trying to find a formula:

1. Go to your local good hardware store, and find some "MEK" (Methyl Ethyl Ketone).
2. Take a small amount of MEK (perhaps 50 ml or so, it goes a long way), put in a beaker or small glass, HDPE or LDPE container.
3. Add small chips of polystyrene "Styrofoam" (I use the dense stuff they use to insulate houses, but probably any clean stuff will work, e.g. packing peanuts.). Keep adding them while stirring until you can tell that the solution is reaching saturation. I stop before they stop dissolving all together to avoid buildup when the solvent evaporates with time.
4. Transfer to a glass jar with metal cap (the best), or (not as good) a film can container. Note that "clear plastics" will dissolve - don't use them.

To use, brush on, let it dry. Then sand with 220 sandpaper and repeat several times more. Your goal is to sand off as much as you put on, until it is smooth, not to build up a layer of plastic. You want plastic impregnated wood, not plastic covered wood.

Obviously don't use on plastic model rocket fins, - it will dissolve ("craze") them, and have no benefit.

Now for the disclaimer:

Like many solvents and paints, MEK is not the most pleasant stuff to work with. My official legal opinion is that no one should play with any chemicals that you wouldn't eat for dinner, so... do this project at your own risk. MEK is very flammable, not good to breathe, and anything beyond incidental skin contact is not a good thing. This doesn't mean that kids should be banned from using it, but certainly it is in the class of using, well, butyrate dope, enamel paints, and thinners. The MSDS sheet for MEK can be found here. ... so don't sue me.