How to varnish a floor made of chipboard?
It was mentioned to varnish it
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Really no way to waterproof it. You can seal it to protect it. A marine grade varnish would give the best protection but it does yellow over time. Several coats of a water based poly would also work and does not yellow.
There is specialist waterproofing paint for best result or use waterproof PVA which dries clear.
Marine epoxy like a top deck ... https://www.amazon.com/RESISTANT-MARINE-NON-TOXIC-FIBERGLASS-LAMINATING/dp/B00BF1XWFI