I've always just used the highest concentration of alcohol I can get. Usually, that is 90%. I used to be able to find 100% ethanol, but it is harder unless you are going to chemist supply or something.
The higher the concentration, the better it will clean the resin and gunk right off. It's really amazing really.
Then I just use a small wire brush to get up in there to wipe it off. If you can't get to spots, use salt or something that will brush it when it shake it around. Just a little friction doest the trick if you have enough alcohol on it.
I don't consider simple alcohol as a lot of chemicals. You can get it at drugstores and works great. Then just rinse with water and you're good to go.