Latex-based or polyurethane-based paint is one of the most accessible forms of waterproofing plywood. There are plenty of colours to choose from, which can be applied using a spray gun or painted with a roller. You must apply two to three coats of paint for maximum effectiveness. Latex paint is also one of the cheapest methods. However, you will need to repaint it often. You can use latex-based for indoor or outdoor projects. Coating your board with latex-based paint can give your board a lovely, complex finish, but you will lose the natural aesthetics.

Lacquer and varnish as a sealant are perfect for both indoor and exterior use projects. It can be applied using a brush, roller or spraying to achieve a finish to your liking. The brilliant thing about polyurethane varnish and lacquer is that they let the natural veneer grain become more prominent, giving you that natural look. Using varnish and paint to seal plywood provides not only an enticing colour but also water protection and UV protection; however, there is a possibility that this type of sealant can yellow over time. 

When choosing your lacquer or varnish, find one that matches the plywood veneer you're working with to give you the best results after sealing.

If your project is used indoors, you could try using wood oils. Due to the porous nature of wood, the oil penetrates and soaks into the surface veneer. This type of sealer gives the wood a natural look and can be applied by painting on or using a cloth to rub the oil into the wood. You will need to apply 2-3 coats of oil, as the plywood will fully absorb the first coat.

This type of sealant works best for outdoor projects as it offers up a bit more protection. The best thing about an epoxy sealant is that you only need one layer to get the best protection. Depending on the type of epoxy you use, you will have an outcome that retains the natural colour of the veneer, or it will slightly tint it. You can apply this sealant using a paint roller or paint brush, ensuring that you keep the strokes even throughout.

Another cheap method of sealant is a water-based paint. This sealing method is not so standard as it's not as long-lasting as the other methods. It is also possible that this paint will need to be touched up more often, but it is cheap, quick and will still protect your plywood from moisture.

It's worth mentioning that it is not advisable for water-based paint to waterproof plywood for outdoor use, as the harsh elements, either UV rays or continuous rain, will wear down the paint within a few months.

If you're looking for the plywood that you're confident, even after sealing, will have the most protection against water and moisture, then perhaps you should consider marine-grade plywood. Marine plywood is a type of plywood that is more water durable than regular plywood. The glue used to stick the veneers, or ply, is waterproof, which means it won't de-laminate when exposed to moisture or water. Marine plywood also boasts that there are no gaps between the plys. Gaps weaken the overall structure of plywood and create a path for water or moisture to get inside the plywood sheet, which can cause the board to weaken from within. The surface veneer of marine plywood is also from a wood that can withstand the elements, thereby giving your plywood a longer life.

As the name suggests, marine plywood is often used in boat building. So, not only does it have excellent waterproofing capabilities, but it's also hardwearing and can take on impacts like a pro boxer.

One of the reasons why plywood is often used is because the edges provide a lovely effect. If this is the main reason for using plywood, then you should consider getting melamine-faced plywood, which means you will only need to seal the edges rather than the entire plywood sheet, thereby saving you time.