I use a soft 6 inch sanding pad on  angle sander with 60 or 80 grit paper . The object is to get it fair. Any ridges will leave air under the glass. Same with cracks and small holes (trap air) Glass cloth will hide nothing.The cloth must lay flat. I used thickened epoxy for cracks and holes. I use thicksill  from Fiber glass hawaii or West systems 410 filler. Insides I sand 80grit. Exposed I sand to 120 grit. Exposed gets 120grit with a random orbit and then finish sanded 120 by hand with a soft pad in the dirrection of the grain. Hand sanding with a sanding block is a lot of work. Better you use a random orbit sander (5 inch) with a medium or soft backed pad. You can get the backing pads from automotive stores or make them yourself. After sanding I seal the wood with Zinsser Bulls Eye SealCoat to keep the wood from soaking up to much epoxy resin. Used to seal with epoxy but the wood just soaked up to much resin. I always glass my first coat on a dropping temperature to avoid off gassing. You can heat your hull with a couple old electric blankeds or heat lamp.(doesn't take much)

If the gap was large I cut pieces of matching cedar to fit the opening.  For smaller gaps, I used a syringe with epoxy/wood flour.

Rich

Thanks for the replies.  I think I chose my words carelessly.  I realize that wetting out 4oz will show everything underneath.  My point was, that on the underside of the deck, my thought is that wetting out the cloth will fill small cracks.  By small, I mean the width and depth of less than 1/16".  I intend to sand it fair.  Once this cures and the deck is attached to the hull, and the deck top has been extensively sanded, I thought I would roll on a couple layers of unthickened epoxy in hopes that it would seep into whatever small cracks remained on the top.  If unthickened epoxy does not fill the cracks, I will fill the cracks with something else.

I fully intend and expect to put more effort into the top.  I do not want to short change my effort on the underside, but do not want to do a lot of work that will not be seen.  Any info/advice about the different levels of effort expended on the top versus underside

I'm sanding by hand because my 6" (5"???) Dewalt random orbital sander's pad is too stiff for the tighter radius of the underside of the fore deck.  I'll see if I can find a softer one...

Thank you.

Paul in Phoenix

When you wet out your 4 oz glass cloth any voids or cracks will end up as air pockets ,they will not fill with resin. If you used enough resin to fill them it would float the cloth not good. Fill them before glassing and save a lot of trouble. If I were hand sanding I would use a soft block and 50 grit paper, you aren't going for looks on the inside.You don't want any air pockets or floated glass cloth , they are weak spots. If you sand . As soon as you finish the glass cloth (laminating) When it is dry enough to touch do your fill coat of resin  .If it is hard dry then you will have a bonding problem unless you sand. I do all my glassing in one operation . On a larger boat it can take a couple days with cat naps between coats.I can save a lot of sanding time by giving up some sleep ! For the inside of a deck you will not be giveing up more than a couple hours between coats and will save you from haveing to sand.