Maple Veneer finishing

Hi and welcome.

No, maple doesn't require a grain giller. It's a closed pore wood, and you only need to grain fill open-pore woods.

It's fine with both water and spirit based stains (turps is fine).

It's fine with all clear finishes, poly, acrylic, nitro, shellac etc.

The main thing to watch out for and remove are glue marks. These are where the factory don't take enough care to remove the glue used to stick the veneer on where it squeezes out at the join line between veneer pieces and round the edges of the veneer. And you occasionally get glue fingerprint marks.

Before you do anything else, look for those. Best to use turps or a turps substitute to wet the veneer surface (don't go mad, but a wipe with a well-damped cloth should be sufficient). The turps should darken the veneer, but where there are glue marks, the turps isn't absorbed so it stays light. Check in a bright light for best contrast. Hopefully you won't see any, but if you do, they need to be removed or your stain won't take in those areas and it will look patchy.

You can use water but water will raise the wood grain and the veneer will then require a light sanding.

You need a glue remover like Goof-Off, which contains acetone, or else just use pure acetone (avoid the cosmetic removal varieties that include oils). A combination of rubbing with this on a cloth and brushing with a soft (brass) wire brush normally does the job.

Avoid sanding the veneer if at all possible. It's very thin (starts off at 0.7mm or so but is sanded at the factory so is generally only 0.5mm on the guitar), so any sanding should be minimal, with the sandpaper dragged along the surface to remove loose fibres, and not pushed down with any force.

The only time you may want to use some grain filler is around the very edges at the ends of the guitar where a dull router bit can pull off the end grain when they do the binding routing, but you only need to fill where there's missing wood. This will require sanding back level once the filler's dried, so just be very careful. It's OK to sand the binding.

Which brings me to binding and stain. I strongly recommend masking off the binding when staining. The binding the factory uses develops hairline fractures where its bent round curves. The tighter the curve, the more and deeper the fractures are. You can't see them with the naked eye, but they are there. Water-based stain tends to sit on the surface of the binding and can normally be scraped off (though it may penetrate a deep fracture), but spirit-based stains have less surface tension and wick into all the cracks. I've had to paint binding before to remove the marks left behind. So covering up the binding before staining can save you lots of anguish. Vinyl pin-striping tape (3M make a good 3mm-wide tape) is good for the top of the binding as it's easy to follow the curves. However for the sides, I find it always pulls away from the binding in concave sections, even if you apply it in small lengths without any tension, so I use paper masking tape there - modellers masking tape which you can get in sets of different widths.