About 75% of the recent discussion(last half dozen+ posts) is incorrect. No one should feel bad about that as the topic is complicated and convoluted and made no less so by by product descriptions and inconsistent terminology.
(I was in the business..as a product development chemist..designing styrene-acrylic polymer backbone structures..basically a performance need was identified and I'd design the polymer backbone that could meet those needs. That polymer backbone was then used in synthesizing emulsions that were formulated water-based coatings.)
Some reference reading..such as it is..
https://en.wikipedia.org/wiki/Emulsion_polymerizationFolks seems to be tripping over the word "latex" as in their head they're thinking "latex rubber". Years ago people shorten the term "latex rubber" into "latex". The world would have been better off if they shortened the term into "rubber". While the milk that is tapped from rubber trees is indeed an emulsion, as is cow's milk and TB2, for the purposes of this discussion..forget latex rubber as it doesn't apply to the line of products being considered(adhesive primers).
Any water-based paint, primer, or stain you buy is, at it's core, an emulsion. The words "emulsion" and "latex" can be used interchangeably. Neither of the words emulsion or latex suggest, or have any relation to, what chemistry(urethane, alkyd, acrylic, oil...) the emulsion/latex is comprised of. An emulsion/latex is just a physical state/conformation of some material.
Another wiki(it discusses two liquids..you can also have a liquid-solid emulsion..of specific interest in this discussion):
https://en.wikipedia.org/wiki/EmulsionAcrylics, as a class of monomers, are inherently brittle materials. Among their many unique attributes, relative to other monomer-chemistries, are high gloss and superior exterior durability. They may, or may not, offer superior adhesion...it depends on how the polymer was designed/developed and what product requirements were being met in that development. Every base-chemistry (urethane, alkyd, acrylic, vinyl...) used in the coatings industry has attributes where they excel beyond the other chemistries. This leads to the need to select the right product/chemistry for your application.
Relative to this discussion, I'd avoid any material/coating based on PVA..poly(vinyl-acetate) as it's the cheap, garbage chemistry of the coatings industry.
For a topcoat (exterior durability), acrylics will be the best.
Urethanes are known for toughness and durability..and "ok" exterior durability...good enough, but not great.
The various chemistries are, as a rule(meaning there are exceptions), incompatible with each other. Incompatible meaning they can't be blended in the can nor topcoated..laying down a urethane base coat and topcoating with a waterbased acrylic will most often lead to a mess and flaking, non-adhering topcoat.
"Primers" are different class of products as they are designed to offer great adhesion to the substrate as well as subsequent topcoats...of possibly different chemistries. If you want a universal primer-sealer..use shellac. I has near-zero exterior durability, but almost nothing beats it as a conversion coating..sealing in any base-coat and offering great adhesion to near-any subsequent topcoat chemisty. In the above example..put down a urethane basecoat, spray it with shellac, then topcoat with acrylic..the acrylic will adhere well. In practice..there are better options for a trailer exposed to the elements.
In your search for an adhesive primer, look for coatings(product descriptions) that describe the attributes you want in a
basecoat..like superior wetting and adhesion to various substrates as well as superior adhesion/compatibilty with various topcoat chemistries, and then select a topcoat for the attributes you need next, like exterior durability, color-fastness, adhesion.... Don't worry so much about superior exterior durability of a primer (beyond being designed for "exterior use") as the topcoat addresses this requirement much better than any primer ever will.
If you try to get all the attributes you want in one coating..you'll get a coating that does everything you need in a sub-par fashion...just how it works..
by John61CT » Fri Aug 30, 2019 9:19 am 
… Might as well get a FEMA trailer if we wanted to be poisoned …. Just saying … 
