As Zandit says, you don't need to grain fill the top. Maple has a closed pore grain structure (unlike something like mahogany) so doesn't need filling to stop it soaking up the finish in an uneven manner. But sometimes the thin veneer splinters slightly at the edges - mainly on the end grain. So look at the top where it meets the binding around the heel and neck and you may notice areas that aren't smooth. If not, great. But if there are a few small rough areas in the veneer, then I'd just grain fill these areas. It's a lot less work for a start!

But the mahogany body and neck will need to be grain filled. You have the option of using a neutral filler and then staining, or you can use a coloured grain filler (or neutral with some stain added) to combine this operation. Gibson have used a number of different tints over the years, from cherry red to a dark brown, but you may want to go with something completely different.

Try and find a water-based grain filler. A lot are naptha or mineral oil based which don't take stains well (most act like a varnish and seal the surface). Stewart Macdonald, the best known US luthier supplier ( http://www.stewmac.com/?gclid=CjwKEA...sxXBoCGKfw_wcB ) sell a water-based grain filler, or you can try and find a product called Timbermate - which is Australian and used by most folks here, but I know it's available in the UK.

As you are staining, you won't want to use a sanding sealer first. Grain fill the top (if necessary) and sand smooth - a maximum of 240 grit, and maybe a quick drag over with 320 grit. The mahogany body and neck will need to be grain filled and stained. It's recommended that you do the grain filling exercise twice on the body and neck (but not the top unless it really needs it) to get it really smooth.

Make sure you let everything dry for a couple of days after staining. You don't want to trap moisture under your clear coats.

Note that if you plan to paint the headstock face black or another solid colour rather than stain it, then just grain fill it, then maybe a couple of clear coats to seal it which are then rubbed flat and then spray the headstock the solid colour.

Then you can start on your clear coats.