Undercoat vs Primer: What's the difference?

I see these two words are seemingly interchangeable, but when I look at product MSDS they are often quite different. What gets used on, let's say plywood with the intention to finish it with a water based paint? The synthetic latex-based primers seems to make claims that it's for prepping surfaces for adhesive, not for painting.

I'm going to Masters first thing tomorrow morning, but if the staff there are anything like Bunnings, then I don't expect any assistance will be available.

Will this do the job?

https://www.masters.com.au/product/9...-pro-primer-1l

Although there is some overlap I always thought primer was more for raw surfaces (concrete, wood etc) whereas an under coat is any coat that's under a top coat. A primer can be be an undercoat and even if 2 top coats are used then the one underneath is an
undercoat. Sometimes you will even see a product sold as a Primer/undercoat.

A good primer will seal a surface properly so that it takes and bonds an under or top coat in the longest lasting manner possible especially if weather is involved.

Purpose made Undercoats are sometimes used to mask another colour of paint or bind paints of different type to each other.

Many years ago I recall using 4 coats of top coat white to cover a deep blue coloured wall. Later when I spoke to my FIL about it he laughed and said "shoulda used an undercoat".

Primer and undercoater serve different roles in a paint project, but much less so with the products DIY's employ, such as low end acrylics and alkyds.

A primer is a "tie coat", usually between a substrate or previously painted surface and a yet to occur subsequent coating. It's used to help promote adheasion, fill imprefections, fill porous substrates, resist corrosion, create a sealing barrier from (possibly) a difficult to paint surface to another, resist temperature swings, improve elongation properties, etc.

An undercoat is often a "bulking" coat to build film thickness, offer opacity, seal down filler(s) possibly, act as a "glazing" compound for pin holes and other minor imperfections.

Again, for most DYI'er projects a good quality primer will perform the tasks you need, but on the other hand, you might need a primer and undercoater if painting some raw mild steel plate, with a water borne paint system (for example).

Personally, I haven't had a real need for this two step approach to priming and undercoating in many years, mostly because of epoxy based primers. A good epoxy primer will answer both needs and you will not have compatibility issues, regardless of the resin system used in the top coats. The only time you really need to worry about an undercoat is, on difficult materials and substrates, in which case there's usually an application specific primer, that solves the problem, such as an aluminum etching primer.