cavelamb himself wrote:

The Wagner Power Painter was my first choice, but even thinned 50:50
(way too thin for this stuff) the WPP would gum up in less than a
minute. It was a prolem withthe pump piston - not the spray head.

I have similar annoying experience with the Wagner PowerPainter.

The power roller on the other hand works really well with even
thick paints, but I'm not sure I'd consider painting a plane with
it.

When it comes to PolyFiber, read those instructions VERY VERY
carefully. The first few coats are required to be brushed on
to get the mechanical penetration of the fabric weave.