The trick to doing this is just knowing which grit sanding paper to use.
Yes. I did say sanding paper.
What you will want to do is first scrape off any of the buildup you can with a plastic knife or something similar.
Once you have the big stuff off you can use some 800 grit wet and dry sand paper to get the majority of what remains. Try not to sand all the way through the deposits but get most of it off.
Next you will need some 1000 grit wet and dry sanding paper. While keeping the surface, and the paper wet at all times ( its gonna get a bit messy so have some old towels on hand to mop up water with) sand off the last bit of the deposits.
1000 grit sand paper is the secret weapon of high dollar car detailers. I'm sure that you have seen shiny black paint jobs that you can see the minute scratches in that are left by a towel. Well 1000 grit will remove them. This stuff is used to polish a paint job. It is very fine and to the human eye any surface it is used on appears polished to a fine degree.
Once you have it all shined up again I recommend that you apply a few coats of a good polymer based car wax. I have had excellant results with TR-3. This will allow you to remove future deposits easier.
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