Using acrylic has several advantages: 1) the acrylic protects the shingles from the effects of the sun 2) the acrylic can perform some weatherproofing against leaks  3) there are many colors available 4) the work can be performed by the homeowner on easy-to-stand-on roofs and 5) if you already have two layers of shingles, this approach may delay/avoid an expensive tear off project

Once the residential roof shingle surface is swept of any loose debris,  the application of the acrylic coating is quite straight forward. No primer is needed. Acrylic coating possess more "body" compared to paint, therefore, a slight alteration in the painting technique would help.   One suggestion in applying the acrylic is to brush the acrylic up and down the shingle. Finish the work by pushing/pulling the brush horizontally so that the acrylic does not collect along the overlap.
1) Priming the wall before the start of the coat
2) Start the scrylic coating

3) Fibre mat for cracklines found on the wall
4) Complete the acrylic coating on the wall
5) Complete the acrylic coat and exposed membrane system