Painting - Painting Cedar Shake Siding?

Older house with cedar shake siding has been painted numerous times (always primed with an oil-based primer and painted with Sherwin Williams flat latex) but the siding typically will not hold the paint much longer than 3 to 4 years. The paint does not bond and comes off in large strips.

I've been told that a power washer will knock off shingles so I typically scrape the affected shingles by hand.

Is there any way to improve the bonding of the paint to the siding such as different primer, different paint, etc.?

Thx...

I would wash it down with tsp, powerwash it, scrape the damaged areas. The power washer wont harm a fly if held far enough away. If too close it will blow holes in rotten wood and raise the grain in good wood. Keep it about 10 inches away with a 2500psi or 12" with 3500 psi and it will take off loose stuff and loosen poorly adhering paint. It will also wash off the tsp (dont let it completely dry out). The wider the fan the less powerfull it is also. I would check to see how it absorbs water by squirting it on and check for it beading up (maybe someone watersealed it very well). If it absorbs ok. prime it with pittsburg paint's "permanizer plus" a clear wood stabilizer that has excellent penetration and is water based ($40/gal.). Then paint it with a satin finish as they have better elasticity in general. I am assuming the paint down to wood is peeling, if the latex is peeling off the oil based primer thats different. Do tell