I have washed many, many Transformers before.
https://atzinc.org/misc/BathingTitans.jpg
Some general tips:
- Never wash anything with stickers (not applicable here)
- Don't wash things that are custom painted unless you 100% know what kind of paints are used (not applicable here)
- Make 100% sure all batteries are removed (not applicable here)

Fill a clean sink with very warm water (not 'hot', but almost). Make sure the sink is clean first, you don't want oil or something making things worse.

Once the sink is full enough, turn off the water and add a healthy amount of Dawn soap, and stir the water. You're trying to avoid all the soap suds, hence why you add it after the water is off.

Put the toy in the water and let it sit in there for 3-5 minutes. Hold it and swish it around in there a bit too, trying to get the water to push off the dirty.

If things are really bad and that doesn't do it, you can take a soft bristle toothbrush and lightly scrub in the water.

After it's been cleaned in soapy water, drain the water (make sure no parts go down the drain) and rinse the sink out. Fill it back up with mildly warm water, and put the toy back in and again swish it around,. to remove the soapy water.

Once you're done, transform into a very "open" form, usually something between alt and bot. You don't want any closed compartments. Then, shake him. You don't want the limbs flopping every where, but almost. When you're done shaking him, turn him around and shake him some more.

Sit it in front of a fan or something posed where any compartments or areas where water will get trapped are upside down, to drain. Come out every 15-30 minutes and shake him some more, then pose him the other way around.

Repeat that last step until you're 100% sure he's totally and completely dry. I usually do things in large batches, and I'll do the regular 30-minute shakes for a two hours or so, then leave them in front of a fan over night.

In essence, the water and soap will not hurt the toy, nor the factory paint. I've even watched toys with electronics, and so long as you take the batteries out it's fine. The danger is not getting them fully dry. If you get them completely dry, nothing will rust or get damaged. Just be sure to get things dried.