Marc, I have to disagree. I am sick to death of the manufacturers trying to turn this into a rich person's hobby, as they did with model railroading.
Anything a person can do to keep their
hobby costs down is a good thing. The $4 saved by using an off the shelf thinner (as opposed to an overpriced proprietary thinner) is the special set of decals, or a detail part that will add much to the builder's enjoyment of the hobby.
Frankly, I'd rather spend that money on the paint—which, at least for the better lines, is worth what it costs. Tamiya= ~$960/gallon; Humbrol: ~$460/gallon based on prices for the 1/3 and 1/2 ounce sizes, respectively. These prices may seem outrageous, but they are not. (Inflated, yes; exorbitant, probably not. There are good technical and economic reasons why hobby paint is so expensive.) Economizing on paint just doesn't work. You can buy an old, cheap kit and make it a prizewinner with enough effort and ingeniuty. There is no way to make an inferior paint perform well.
The proprietary thinner may be no more than repackaged mineral spirits—or it may have been specially formulated for that particular brand of paint, or both paint and thinner may have been formulated to give less than desireable results unless both are used together. Considering modern business practices, either the first or the second are most likely, and you are paying approximately $30 per gallon for
solvent—that's the price per gallon of a premium interior latex!
That being said, using the proprietary thinner for thinning before spraying has the justification you mentioned. But for cleaning, when you blow the stuff out the window in profligate amounts, the cheaper you can get it, the better.
Bear in mind that the cheapest mineral spirits you buy at Home Depot, etc. , may also not be the best quality in the world—not all solvents are created equally, and the stuff sold for cleaning brushes or thinning paint by the pint, quart, and gallon may have permissible contaminants that may not affect its performance when used in large volumes of paint, but will play havoc with hobby paints, where the reduction percentage may range up to 50 percent. Discount houses, in particular, will stock the cheapest stuff they can get, as well as better quality materials. When buying non-proprietary solvents for cleaning or thinning, it is best to go with a premium brand, or at least one with a reputation you trust.
<climbs creakily down from his soapbox>