Cleaning the interior hull of dark film

[PNM]

While pressure washing one of my engines I noticed the "non bilge" areas under the plywood engine compartment floors are covered with a (probably 40 year old) dark film. Some of the areas are hard to get to.

Recommendations about a product or process that will help me clean these difficult areas to good condition would be appreciated.

I am considering removing the plywood floors - a little at at time - and replacing them with non-corroding mesh or grid to allow regular cleaning.

Thanks for your help.