Thanks for the advice.
I have a couple of hairline cracks (probably about 1/16" to 1/8" tops) along the mortar joints here and there.
Is there a filler you'd recommend?
If you have any cracks that are 1/4" or larger I would be a little concerned as there could be an underlying issue. Since you said 1/16" to 1/8" the primer/elastomeric paint should help fill them.
If you are anal about your projects, you could look at a mortar repair kit like the one I've included below or another non-shrink grout (sika, quikrete, etc.). Most products for crack repair require a minimum of 1/8" for it to bond and be effective. Each will have a different cure time before you can paint, you'll find that in the product data sheets on manufacturers websites. Personally, I wouldn't worry about it unless the cracks were larger.
https://www.lowes.com/pd/QUIKRETE-Zip-and-Mix-FastSet-Repair-Mortar/3472653