Regarding how long a "grind" will last, I think it really depends on environmental conditions its exposed to. A covered canopy carport type structure is the only way to go in my opinion but it looks like you have a lean to structure to help with that. Making sure rain water doesn't get in will prevent contamination and runoff from trees will prevent algae/mold/mildew from developing. The reality is that you could spend many days grinding only to get to the actual glass work weeks later. The biggest thing is making sure there are no contaminants that have entered into the grinded voids to prevent the best mechanical bond possible. Polyester resin has the weakest mechanical bond with Vinylester being second and epoxy being first. However, polyester is what boats are made of still to today and do just fine with rebuilds as long as the proper prep is completed. Another thing to consider is your finish because if gelcoat is preferred, you will need to be careful not to use epoxy where you plan to finish with Gelcoat. General rule of thumb is that epoxy finishes will stick to polyester/vinylester based resins but Gelcoat (polyester based) will not adhere properly to epoxy resin. Lastly, although heavy woven doesn't need to be grinded flat to create a bond, if there is any leftover wax from the original layup, this could contaminate the repair layup. I always start with a pressure wash to make sure all the glass is free of dirt and debris. Once the woven roven is free of debris you can wipe it with acetone to remove any original wax and then grind away. I personally believe wiping with acetone LAST contaminates the "teeth/grooves" that was created from the grinding and the last step should always be grinding/sanding with heavy grit(36-80) . I also find that a dual action compressed air sander helps a lot to get everything ready right before you start to lay glass. Hope that helps and don't hesitate to ask questions.

Just wondering if you figured out the operation of this cap.  My 2001 215 Sport has a similar one and I'm thinking its not working or maybe I'm overtightening it.  On a day in the the 80s (or warmer) my carbs will weap fuel and drip om the ground.

For the hull sides, if you're next to the transom where the new glass will overlap from transom to sides as Rick described just be sure it's ground clean and free of things sticking up that would cause an air void in the new glass. But you don't need to grind the weave of the heavy woven glass out.
grind the ridges flat - do you mean at the ends of the new glass? If so, and you plan on overlapping with glass later on, then yes you'll want to knock that ridge down to once again keep from getting an air void under the new glass.
Best to layup the new glass wet on wet if you can stand the work hours, that would avoid having to grind between layers.

oops
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Thank you gentlemen. On the hull sides on the heavy biaxial I assume it needs to be ground flat in relation to the significant relief/texture of the material itself.... grind the ridges flat? For that matter, how long does a grind last assuming it gets nothing like grease or dirt on it?  Ya'll, for the record,  are ridiculously helpful and gracious with your time and knowledge.  Thank you.