Water-based polyurethane wood sealant (PU) is the simplest, safest and most foolproof option IMO. It's the exact same stuff I use commercially to seal my own blades.
It's not very expensive stuff to buy, and is available from almost any (every?) decent hardware store.
It comes in matt, satin and gloss finishes - any of which will work fine (though gloss can sometimes require an extra final light sand afterwards, otherwise the rubber glue won't stick to it). One small 250ml tin is plenty!!
Don't bother buying an expensive brush - you really don't need one. The corner of a clean and dry lint-free rag will do.
The stuff looks like milk, and can be runny as water, or else a bit more honey-like when fully stirred.
On that point, be sure to mix the PU well before use. Be sure to use a wide and clean stirrer, as there will be a thick layer of white sludge on the bottom of the can.
Don't shake the can to try and mix it, as this introduces air bubbles, which can ruin the finish.
Some brands of PU are thinner than others, and sometimes the viscosity can vary considerably even when sticking to one brand.
Don't use anything coloured or dyed to stir your sealant - PU picks up dyes and colours quite readily... A single, wide, pine popsicle stick is perfect.
Make sure your blade is clean, dry, and free of dust, grease, and fingerprints - any of these will gum up your finish.
Wear disposable gloves - this stuff ain't toxic, but you don't want it staying on your skin either.
Dip one corner of your rag in the sealant after mixing, squeeze out any excess so the closth is damp but not dripping, then carefully wipe one thin layer on your blade's playing surfaces.
Put your blade aside to dry - on a really hot day the first side you coated can be touch dry by the time you've finished the second side. Room temperature is the ideal temperature really to control the drying process.
To avoid using too much with each coat, first give one side of your blade a consistent coating, so that it actually looks wet and glistens in the light. Then wipe off the excess with a part of the rag that's still clean and dry. This should give you the perfect coat thickness.
One un-sanded coat is usually sufficient, myself however I always lay down two to be thorough (I'm an anal perfectionist when it comes to my blades
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Once the first coat is fully dry (drying time varies depending on ambient temperature), and if you intend to give it another coat, I recommend you give it a quick and VERY light sand with 200-250 grit painting sand paper. Wipe off all the dust and then repeat the above, laying down another thin coat, again wiping off any excess afterwards.
Once it's fully dry you can sand back this second coat too if you like, but if you've done all the above right, the second coat will be so thin and flat it really shouldn't be necessary.
The above process will give you a thin and even coat of sealant over your blade, which is more than sufficient to protect the wood from years of regluing, but is not so thick that it speeds the blade up or affects its playing feel (or exceeds the 0.1mm limit on varnish set by the ITTF.)
If however you WANT to make your blade faster (and don't intend to enter any ITTF comps where bat control protocols are in place), then do all the above, but don't wipe off as much with the dry rag each time, and do more than two coats.
Hope this helps.