Best advice I can give is:
1. Oil, etc. removal with a sodium silicate based detergent .... this
will also penetrate deeply into the open pores of the gelcoat .... and
will also remove the old dead wax. If you can find a sodium silicate
based detergent then use any strong 'caustic' detergent (not lye).
Followup with:
2. bleach wash with a saturated solution of oxalic acid and warm/hot
water .... this will remove the brown tannin and iron staining. To
do this right you have to do #1 above first, then rinse with water then
apply the oxalic.

Use protection when using as oxalic acid will quickly absorb through
your skin and do great harm to your kidneys.


In article , Rick
wrote:

After a beaching with Katrina and the refloating with Rita I am left with a
boat with a brown scum smile on the side of the boat. The residue is from
mud mixed with oil and diesel from the sunken boats that were not so lucky.

Question is cleaning the hull? My thoughts are open at the present time and
am wondering if the groups experience base can help? Right now the boat is
in the boat yard so access is fairly easy.