is it better to use 2 pieces of 1/4 in. birch ply epoxyed together or 1 1/2 in. birch ply with 5 or 6 cotes of varnish? why do most people use 2 1/4 in.? is it stronger?

We use two pieces of 1/4 in ply because we are using a table to fix shape into the board. You can build into your board rocker and concave, plus you can sandwich fiberglass between the two pieces of ply with relative ease.
Plus it is stronger, much stronger especially if the board has concave.

A table is easy to make, may only take a hour to make. You will be able to make multiple board with it.
Use Screws, clamps, or wieghts to hold the pieces together during layup, I prefer Drywall screws and fill the holes in after with floor and epoxy, If your using clamps you need to clamp every 15cm or so, thats alot of clamps.
The actual main constuction time for putting the two pieces together is about 15 to 20 mins, this is the working time of the epoxy.

so if i'm not going to have a rocker or concave because my board will be a light wind board 138 cm. x 47 cm, then i'm ok with 1/2 in. ply? just trying to keep my budget really low. i'm 6'2" and 210 lbs. good size for light wind?

138 x 47 - yes good size

I guess you already know this ? - cut the board with the grain travelling along the length of the board not across it.

bay surfer
could you tell me how do you add the fiberglass?
i am still researching about what i am going to do... i have the design in my mind but i want to make it strong ...i already know that i want 2 6mm plywood ...but that fiberglass thing you say in this last post got me thinking...how do i do that if i want to make oit stronger?

please explain the procedure

Sandwich is easy coat the 2 pieces with epoxy lay your glass down on the wet/slankin epoxy, put the pieces together, clamp middle to the outside to get the air out.
Glass is really not needed though, wood soaks epoxy up, wood fibers are kind of like glass really, thats why 2 pieces of ply epoxyed together are stronger than just one big piece of ply
I do use strips of glass in between (Stringers) to change how the board flexes. For that I use fiberglass drywall tape. It sticks to the wood, wet the ply and tape at one time.

What? No rocker? I would do some rocker even if it's a lightwind board!

BR,
Zob

You can get a way without rocker, but the board will be a lot nicer with it. If you use two 1/4 sheets, you can also leave the edges one layer and make the center two layers. I think about 10 inches at the tip one layer should be fine. I made a board from one sheet (1/2), routed it thin at the edges, and then bent with pressure and water. That’s a lot of work. With 1/4 sheets you can have flex tip without the routing.