Degrease surfaces with the appropriate degreaser
Prior to sanding, ensure that the surface is clean and well degreased. Loose and water soluble dirt may be removed with (warm) water and ammonia. By degreasing before sanding, grease and contamination will not by sanded deeper into the sanding scratches. Degreasing should exclusively be done with multiple clean (paper) towels.
Always use the appropriate degreaser:
Degreasing existing paints/varnishes : Epifanes Spraythinner for P & V
Degreasing bare Fibreglass/GRP : Epifanes Fibreglass Prep Cleaner
Degreasing bare steel : Epifanes Cleaning Solvent
Degreasing bare wood : Epifanes Spraythinner for P & V
Degreasing tropical wood (teak) : Epifanes Thinner for PP Varnish
TIP: Turn and replace (paper) towel regularly. Allow sufficient time to evaporate and the surface to fully dry.
Take personal precautions when degreasing and wear protective gloves/protective clothing/eye protection/face protection.