I've seen people using old etching liquid based on Sodium persulfate. The copper sulfate in a used solution can be used to electrochemically deposit the copper on a PCB.
The results, at least the ones I've seen, are not that great and have a very rough surface. Depending on your impurities it will also lower the quality. But with a pure copper solution it should work quite well.
Another way is using a chemical deposition of solder. There are sets available online for Sn surface finishing of PCBs. I've bought one on ebay a few years back but didn't really use it as it was too messy.
Many PCB manufacurers offer you to use a thicker copper layer 2 oz instead of 1 oz. The proper way is of course to design your traces in a way that they don't need reinforcement.
With homemade boards I also had the problem a few times and I soldered thick copper wire (1.5 mm2) on top of the traces. Imho, this doesn't look to bad if you complete flood everything in solder. With lead free solder it can be a little bit painful and I recommend using enough flux.