Spray Foam Gun Chambers Resurfaced on Sandpaper
Everyone cleans their chambers on sandpaper and it does do a good job at removing small scratch marks.Over time, the chamber will become "cupped" from sanding in a circular pattern and inclined from pushing hard on the front end were the scratch marks are.In our shop, we resurface worn chamber surfaces to new condition. Its not uncommon to have to remove over .0005" to get to a flat surface.
Here is a link to a picture in our site to show an example of a chamber that has both characteristics: http://www.spfdepot.com/product_p/sanded-chambers.htm
To properly sand, we use 600 grit to get it done fast and finish it with 1200 grit. You can get both of these grits from Harbor Freight at a very reasonable price. Push or pull the chamber in a straight line only while holding even pressure in the middle of the chamber and make sure you have a clean flat surface for the sand paper to sit on. Do not rub in a circle - that will "cup" it and pushing to hard on the front part will slope it.