I have a great brown sugar, onion, spices and catsup-based barbecue sauce for pork spare ribs, but when I bake the ribs in the oven, the sauce turns watery and thin. What do I need to remove or add to keep it thick? Thanks.
When you bake meat, certain proteins in it change and contract, squeezing out liquid. That liquid will mix with any miscible liquid around it, of course. The result will be colored like your BBQ sauce, but will be diluted.
If you'd like to have a thick glaze of BBQ sauce on the finished meat, the solution is to apply it late in the cooking process, once much of the liquid has already come out of the meat. Once you've added the glaze, make sure not to cover the meat: you want additional water vapor to float away, not to re-condense and rain on your ribs.
Now, if you'd like to improve the flavor and texture further and don't mind spending more time to do it, you can do a hybrid of those two methods. Cook the meat with the BBQ sauce, tightly covered, until almost done. Drain off and reserve the liquid. and skim off as much fat as you can. Boil the liquid down in a saucepan until it's quite thick, then re-spread it on the ribs. That process takes longer, particularly if you're being really careful about the fat, but the gelatin and flavor in the resultant glaze will be worth it.
It's not likely that your sauce is turning watery. What's probably happening is that the water in your sauce is evaporating and condensing on the (presumably) foil you've wrapped your ribs in, leaving behind the solids of the sauce on top of the ribs.
The condensate will then be mixing with some of the sauce solids, but not to the same saturation as the pre-baked sauce, resulting in what looks like a 'watery sauce', but it's just separated into sauce solids and sauce liquids.
That's why normally rib recipes call for two applications of sauce: one pre-bake to flavour the meat, and one post-back to apply the sauce at the correct consistency.
The other answers here are bang on, but if you've already added the sauce to the meat then I usually add either cornstarch or flour as a last resort to it to thicken it up. There are a few other ways listed here in case they're helpful.