On my first build I used Duplicolor, an auto paint that lots of people have gotten good results with. In my case it never hardened properly. I think the most likely culprit was humidity, heat or both in the original application. I followed or exceeded the directions for drying and curing times, but to no avail.

While I have not done a scientific study of this, I have noticed a lot of folks here, and elsewhere on the web, seem to do their spraying in a climate controlled environment, or outside during periods of the year when the weather is right.

Over time, I have come to use only shellac or products that are water-based. Either kind of paint can trap moisture when the humidity is high. With solvent based finishes, I think what happens is that moisture can get trapped under or in the finish. The surface may harden, but what's under it may not. They will eventually gas-off the solvent but the water remains.

Something similar can happen with water based finishes. However, water based finishes are designed to gas-off water vapor. I suspect that with some water based finishes this the high humidity doesn't stop the gassing off process, but rather just slows it down.

I have found a good rattle can spray paint that is water based and seems to produce a good finish no matter how humid it is here (MTN94). That's good since our average temps range between 27-37C, and I can only spray outside. Unfortunately my luck has not been as good with MTN's spray on varnish that is designed to cover MTM94. The varnish is solvent based.

The clear coats that I use are only spray-able with the right equipment (which I don't have), so I brush it on. I can brush in my garage, which is semi-climate controlled. Hotter and more humid than the rest of the house...but since we only run the AC in the garage when someone (me) is in there it can be relatively warm and humid too. The good news is that all the Crystalac and General Finishes products I have used harden just fine. The bad news is that I need to give them at least them the maximum time that the mfg recommends to cure...and usually a bit more. The worse news is that I have to get enough finish on to be able to sand out the brush strokes...and then I have to sand out the brush strokes. This takes a long time to get flat without going through the finish.

I have had reasonable with stains but only when I have used fairly expensive products from General Finishes and Crystalac. In both cases, also water based. Which means it can swell the wood, so you need to be diligent about raising the grain.

This is a very lengthy way of saying that if you can't control the heat and humidity, you may need to find products that work in the climate you have. That could take some experimenting. Shellac, of course, was developed in India and definitely works there. I'd be curious what people who finish wood there use, or what tips and tricks they recommend. I also suspect that anything that works for you will work for me as well ;-)