I have an old car that I painted recently. The paint job wasn’t great because the paint wasn’t glossy and mostly it wasn’t smooth. I want to find a way to cover up the bad paint job by using some sort of chemicals to make it glossy and smooth. I don’t have the required time to sand it back down and start from scratch. Thanks if you tried to help. I might upload a picture of the paint job if you request one.
If you’re just looking for a 50/50 paint job, (looks good at 50 MPH or 50 feet away), then roll on some Rust-Oleum paint.
Tester
Without seeing pictures it sounds to me like the paint is uneven and unpolished. If you’re lucky you can sand the whole car and get the paint even, then sand it with higher grit to get it shiny.
If the paint is really bad, then you will never get it to look good unless you sand it down and start from a smooth baseline.
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How does sanding with higher grit get it shiny? I’m curious.
it will inevitably go through the clear-coat and sand the base-coat, and then the metallic specs will get exposed and will shine all the colors of rainbow… ask how I know 
the only good option is to sand it down, prime and paint again
option with a Rust-Oleum is quite appealing for the 20+ years old car
Clear coat? Base coat? I’m thinking those aren’t on the car anymore, and there’s a thick layer of paint. 
Higher grit is smoother. If you sand with increasingly higher grits, wetsanding at the end, you’ll end up with a shiny surface because smooth = shiny.
Wet sanding removes imperfections in the final paint/clear coat.
You then follow up with buffing and polishing compound…
Tester
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Concur w @shadowfax above, it’s worth a try to sand/buff the existing paint job to get it looking a little smoother looking. Try it in just a small area first, to see if it will achieve the look you are after without causing other problems in the process. If a little is good, more won’t necessarily be better, could well be worse, so stick to just the minimum amount of sanding/buffing you can get away with. If it works in a small test area, wouldn’t be overly difficult to do the whole car. Not even close to what it would take to remove all the paint back to bare metal for a re-paint. Like I say, worth a try.
Sandpaper is graded by grit size. The higher the number, the smaller the grit size. Probably start with a number in the 300’s at least, be very cautious when sanding w/the lower numbers you don’t overdo it, and proceed up to 1000 or 2000 grades. The higher numbers will be wet sand paper, meaning you sand a pre-wetted surface rather than a dry surface. I used to use 2000 wet sandpaper followed by an optical grade diamond grit buffing compound to sand the surface of silicon chips, looking for cracks and other imperfections through a microscope.
Decades ago I was a body man, but not a painter. Years later a bought a Triumph Spitfire which needed paint. I did the prep at my friends body shop. I then sprayed on the single stage enamel in British Racing Green.
The result looked like a pickle; all green and lumpy. I spent many hours wet sanding the fresh enamel. I had to use very light pressure to avoid clogging the sand paper. After much buffing, it looked great. I certainly had put enough paint on it.
Reluctantly commenting but the thing with real enamel is that it cannot be buffed out once the sheen is rubbed or sanded off. The surface of enamel is shiny but once that top film is gone, it cannot be shined up again. That’s why when doing a spot repair with enamel, it needs to be blended in to the surface and not polished. Lacquer and the new paints are different, where they can be sanded and polished.
So you didn’t say what kind of paint you used? Rustoleum, auto paint? But agree there is nothing easy you can put on it to smooth it out. If the paint you used can stand a clear coat, that might help depending on how bad the current surface is. But yeah if you sand it down with 400, then 600, then 2000 grit paper, you will need to machine polish it starting with an aggressive rubbing compound.
I’ve done a little weekend body work and once the paint is screwed up, the easiest way to fix it is to do it over. But take a look at Tester’s links and you’ll see.
Back in 1966 I got my car painted for $20 at a body shop. I did all the prep work, taping, etc. so all they had to do was shoot the paint on. Prices are a lot higher now but spraying paint is the easy part if everything else is done. Before that I actually used my Mom’s Kirby vacuum cleaner with the sprayer attachment (yeah she was very concerned). Or Harbor Freight has throw away paint guns and compressors. So it doesn’t have to cost a whole lot.
Friend of my dad restored an MG-B, finished it off with a nice red paint job in his garage. Once done he pulled down the overhead door, freeing all the sand and dust from his restoration to fall on the fresh, still wet paint…
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