Here's something I picked up some years in the past, relating to a subject frequently discussed on various boatbuilding fora:

Scarfs are superior to butt-joints, right? More work, but the results are fair, smooth and invisible on painted surfaces. Doing them with a block plane is the time tested, fool proof method. It is almost like woodworking, too, except off course the medium is not wood. So, to me the charm wears off quick.

A while ago I built a drift boat to the design of Tracy O'Brien. Not my type of boat, but a friend wanted me to do it for him. The good thing about that project was the scarfing jig concept that O'Brien included with the plans: Simple, inexpensive, effective. Home made in less than 30 min. Fits any circular saw. Does up to 9mm ply. So perfect for our boats.

I've used the old Version 1.0 for almost a decade now, and it works really well. With a steady hand there's no clean up needed, and the leading edge of the scarf is feathered to nothing.

Unfortunately I don't have the original building instructions any more, so these pictures and a short description is all I can offer. Proceed with caution!

Go ahead and remove the blade guard completely from your saw. Actually make sure it is unplugged first, then the guard business. Get a carbide tipped 8" blade from the hardware store to replace the standard 7 1/4" blade, or else it won't do 9mm ply. Most saws have room for this conversion.

Now make the screwed together fence shown in the pics. The angle in relation to the blade is 6 degrees. Rip a 32" long scrap of 'two by fir' on the tablesaw with the blade set at 6 degrees. Cut the piece in half and assemble it like shown with a row of screws, neatly predrilled and countersunk. You now have a 16" long fence, with a tall, angled bevel on the front face. Clamp it to the baseplate of the saw, with the outside corner of the fence just shy of the blade where it meets the baseplate, and bore a couple of holes for #10 machine screws to hold it in place. Done.