You need to determine if you are dealing with crazing or sander marks, it is hard to tell from the photos. If you have crazing it needs to come off but that doesnt mean the whole bottom,just the crazed areas. Barrier coat is just a paint full of solvents which evaporate away on curing and WILL NOT take care of crazing. I personally have a boat with bad crazing and i will be peeling it next summer. It bears mentioning that in the areas where you go through the gelcoat you need to seal the raw glass with proper 100% solids epoxy
before applying the "barrier" coats. You dont need to use the epiglass resin that interlux
pushes for this purpose, any epoxy
such as West or Systems Three will do the job the same.