Restoration process of old gelcoat
1. use a strong caustic detergent (such as a sodium silicate based detergent) to remove oil and 'organic' staining including mold/mildew, then rinse2. followup with a mixture of oxalic acid and water to remove/bleach out rust and tannin (brown) stains3. Flat sand the surfaces with 1000--->2000 grit wet and dry sand paper (in a sanding block) using a few drops of dishwahsing detergent in water as the 'lubricant' to remove the surface layer of oxidation ... and make the surface FLAT again. 4. Get a variable high speed autobody shop polisher with 3M knobby foam polishing bonnet. Start with 3M coarse fiberglass polishing compound for the worst areas, then follow up with 3M "Finesse-it, then Follow up with 3M "Perfect-it". Dont mix grits with bonnets (foam bonnets may be washed). The surface at this point should be back to nearly show-room NEW condition, then wax with a natural carnauba wax.Keep the power buffer 'moving' at all times so you dont 'burn through' the gel. Work in 2ft. by 2ft. squares when powerbuffing . Be especially careful on buffing out the gel on corners, etc. If the gelcoat begins to show 'thinness' then the boat needs to be painted. Also, before you begin this restoration get a cheapy pocket microscope from Radio Shack, etc. and examine if the gelcoat has become 'alligatored' .... teeny cracks in the surface that look like a dried out mud puddle. If alligatored, then the only remedy is is sand deeply down into the gel and then paint it. BTW .... even if you elect to use 'temporary' acrylic coatings such Polyglow be sure to completely clean (as above) the gelcoat BEFORE you apply it. Use the remdey above THEN use the cleaners recommended by the temporary acrylic coating manufacturers .... for best results. Hope this helps.