Ok - so I recently bought a World's car to restore. Following the guidance of some previous posts, I thought I would give the $29 Ultrasonic cleaner a shot, along with a Hydrogen Peroxide solution. So far, I am not too impressed
That's because your using hydrogen peroxide. I use "LA's Awesome" cleaner in mine from the dollar store, although simple green or other cleaners will work also. Hydrogen peroxide isn't a cleaner. That's used to whiten the parts in sunlight. I would not recommend a cleaner that doesn't have heat either, the heat helps a ton.
Edit- or did instead your post incorrectly? Maybe you cleaned first then put in a bath. I'm unclear.
They work great as a cleaner - I use mine all the time. Mine was an equally cheap one (in the UK) but a different make and although it isn't advertised with anything about a heater, I find if it is left plugged in for about 30 mins then it does heat up. Often particularly dirty parts take several cycles, during which time it has heated up anyway. To aid heating up, only put just enough water in to cover the parts and for cleaning of plastics I put a good blob of washing up liquid in with it.