For starters lemme disavow any notions immediately. This is not how to do it perfectly. This is the quick and dirty method. This is the "good enough no one notices the repair" repair. More fastidious home repairers would get very cross at this method, but I'm not looking to win concourses, just have a bike that looks like someone cares about it.
For the purposes of this, I've just been doing some surface rust in the tank of my bike. This is really easy and comparably cheap to do at home, literally anyone with the following equipment can do this themselves and save some money on panel beating/ spray painting.
The tools I used for this repair were: A small square of #120 sandpaper (like 20 cents I guess, I cut it out of meter length that was worth $2.50), a can of colour-match-by-eye 2pak pearlescent white($32), a can of matching 2pak clear coat ($32 (you may not need this if you're working in solid colours)), some brown paper ($2 a roll), dedicated wax and grease remover (>$20) and general purpose thinners (>$20) a small paintbrush I found and lots of masking tape. So as long as you've got about $150, two spare hours and a sunny day, this is completely achievable.
Before you get started, take a bit of the car/bike/scooter your gonna repair to a professional auto paints supplier. Even if they won't do an "official" colour match (and they probably won't if it's a bike/scooter), they are more than happy to give you the colour swatch booklets and let you make your own decision as long as you promise not to hold them responsible for how well the colour matches the original.
I didn't actually get a photo of the bit I ended up working on, but here's the "before" shot of another repair to the same tank I made:
Just a bit of rust bubbling up from the seams. I chemically sealed the tank so repairing this rust is just about aesthetics and stopping it spreading.
To start with, I ripped in with the #120, I also used a small razor blade to chip off the big bits of rusted paint. Once all the big stuff was gone, I used the #120 to bring the worst affected areas back to bright, shiney metal and feather the sanding out into areas of unaffected paint. It looks like this:
Wiping the area over with a dedicated wax and grease remover cleans other contaminants from the surface, it's imperative that you don't touch the area now with your fingers, lest it all go really wrong. Once the area is clean, it should look like this:
Now comes the fun part, grab the base coat spray can (in my case the pearlescent white) shake it up and spray it into the lid.
You might have to spray a few times times to collect enough liquid paint to brush paint on. It's worth noting that 2pak fumes do cause drowsiness and giddyness if you inhale them. So hold your breath while spraying (no seriously use a face mask). Once you've got the paint collected, use the brush to dab it on. It's important to load the brush up a fair bit and do this in the shade and the cool. Too much light/heat too soon and too little paint on the brush will cause the paint to tack off too quickly and leave ugly brush marks in the new coat. It'll look something like this when you're done:
At this stage it's important to distribute the paint as evenly as possible too. In the dips (where you can see bright metal) is where you want to position the most paint, then get lighter out to the edges. Once you're happy with the first coat, leave it for 5-10 minutes. This is the time consuming part, waiting for the paint to tack off to apply the next coat, but by doing this, you're achieving the maximum amount of adhesion on each successive layer of paint, without having to worry about using a proper primer. 2pak is self-leveling as well, so you'll notice the paint getting smoother between coats.
The easiest way to waste time between coats is cleaning the brush, if the paint is allowed to tack in the bristles it makes smooth applications of the 2nd and 3rd coats even harder. Swishing it around in a jar of thinners and then water washing it gets and good and clean for the next coat.
After ten minutes I went back and applied the second coat. This is what it looked like, I tried to load the brush even more and use smaller strokes to smooth out the finish even further.
To waste the ten minutes between this coat and the next, I got stuck into the tank with the brown paper and masking tape. There are no prizes for neatness, but aerosol sprays can turn corners, so anything that isn't covered by paper is gonna get hit by overspray.
This was my final brush coat. Again, trying to be even smoother with the brush. Now is the part you get to actually use a spray can as a spray can. Once again, leave the project for ten minutes to tack off.
If you're like me and haven't been using a facemask, then overspray will have gotten you good and dizzy by now, so a little trip into direct sunlight does you the world of good. I find it helps to stand away from my intended target and have a few short test sprays to get a feel for distance and pattern of dispersion of the spray can. When you're ready, give your repaired section two or three quick but steady swipes with the can.
The first thing to notice is the how the self-leveling effect goes into the overdrive here. If you were even relatively smooth in the first three coats, the spray coat will even out quite nicely any small inconsistencies. If you are worried about big inconsistencies, before the spray coat, use the wax and grease remover on a rag to smooth down the paint. Hopefully you've got enough on there to be able to spare it. This is the trick, balancing getting enough paint, with getting it smooth enough to look acceptable. It's not a huge disaster if you get it wrong, a rag soaked in thinners with clean the paint off and let you start all over again. It's annoying, but worth it if you're that worried about the finished look.
After the spray coat is done, leave it in the sun for half an hour to tack off again.
Now you're on the home stretch. Invert and spray to clean the first can, then take the second can (the clear coat) and give the repaired area the same quick but steady two or three swipes that you used with the base coat. Then leave it for ten minutes. Repeat this process two more times (for a total of three) and it should look a bit like this:
Now leave it for another half an hour in the sun to dry. If you're incompetent like me and have neglected to use a face mask, you may very well be feeling very woozy by now, so go get some fresh air.
You're all but done now. Pull the paper off to see how well it's blended with the original paint. If it's not great, you can sand it back and try again. If you're really anal about it, you'll notice that there are hard lines in the new surface where it abruptly stops. These are cause by the tape you used. To get rid of them, use a rag soaked in thinners and gently rub from just inside the edge of the new paint, out onto the old surface. This will break down the hard edges and feather them out, to make the repair less noticeable. Remember though, if that thinners gets on your nice new repair proper, it WILL strip the paint. So be super careful
Here's the before and after:
Before:
After:
Totally worth it, right?
Now you're ready to refit that part to whatever machine you pulled it off (if you pulled a part off, you don't have to, the point of this process is you can use it on panels/ parts that are still on the vehicle.) Now take the rest of the day off. Leave the paint to cure and harden over night and the next morning hit it with a polish and groomer/swirl remover (you don't have to, but this will help blend the finish in and it can be done one the "what-ever-it-is" is replaced on it's vehicle).
In my case, this is the finished product:
It's not the best looking, but once the tank is on you'd be hard pressed to spot it without actively looking for it.
