Fiberglass is sold by the yard, and the weight of the cloth refers to the amount of resin required to saturate a yard of the cloth. Thus 6-ounce cloth requires 6 ounces of resin to wet out one yard.
4-ounce cloth requires 4 ounces of resin to saturate one yard of cloth, etc. 6-ounce and 4-ounce are the most common weights of cloth used in the industry. Some vintage style boards use heavier weights and are available on request. Please don't hesitate to ask for specifics.
"S"-cloth is manufactured in a more expensive process, and that is reflected in the cost. It is claimed to be 30% stronger than "E"-cloth. If it is not easily available locally, it will have to be ordered, so there may be a time delay in completing the board.
The glassing process involves saturating the cloth with resin and squeegeeing it onto the blank. The two step process starts with the bottom and finishes with the top. The strength of the glass job is usually determined by the weight of the cloth and the weight of the foam under it.
The standard comp light is 2 4-ounce layers on top and a 4-ounce layer on the bottom. The next up in strength is 6 and 4 on top, and a 4-ounce layer on the bottom. If more strength is desired, then use 6 and 4 on top, and 6-ounce glass on the bottom.
A very strong board would be green foam and a 6/6 top with a 6-ounce bottom. Mack Surfboards usually recommends this glass job for people who are "ding prone" and break a lot of boards, and where board weight is a secondary concern. Do note that a lighter board is easier to maneuver, and is generally better for quick direction changes and snaps. Like the rest of the industry, Mack Surfboards cannot guarantee any surfboard against breakage, but common sense and care will avoid most major problems.