cheap clearcoats

Considering the cost of the model, and the many hours you have invested in building it, I would not risk using anything but high quality clearcoats made specifically for models, such as Tamiya, Valleo, Model Master, etc.  The cost difference is negligible.

Testors Gloss Coat in the rattler is cheap.

I used to by "special" hobby clears...(Model Master). They'd run about 5 bucks for a little bottle of lacquer gloss. I bought a whole quart of Minwax gloss (thin it about 50/50 with lacquer thinner, for airbrushing). I've sprayed over 200 models with it, as well as using some for wood projects, and I still have nearly 1/2 of the quart remaining. I spray the lacquer over enamel paint (no experience with acrylics)...light coats first! Get too heavy, too quickly and the clear will wrinkle up the paint.

is it OK to brush a clearcoat?

There are spray cans of Polyurethane, Acrylic, and Lacquer clearcoats that work fine for models.  They tend to be cheaper per ounce than hobby stuff, but the nozzles are bigger so they do go on a bit heavier, but they do work.

As far as brushing on a clearcoat, it depends on your brush painting skills.  Matt clearcoats are a bit more tolerant of brushmarks than gloss.  Glosscoating with a brush requires pretty good handbrushing skills.  BTW, one nice thing about Polyurethane clearcoats is that several brands offer a nice eggshell sheen.

Both the acrylic and the lacquer clearcoats dry fast.  Most polyurethane varnishes are 24+ hour dry, but miniwax does make a faster drying version.

All of the hardware store clearcoats are available in regular cans and can be thinned with appropiate thinners for use in airbrushes.

My go-to clearcoat is Rustoleum brand in a rattle can. Just as good as hobby brand clearcoats.

fermis

Get too heavy, too quickly and the clear will wrinkle up the paint.

I can attest to that being true. I am building a 1/72 B-1B, and had to repant part of the belly after the grey wrinkled.