Well you have to start somewhere and the Rockey’s Tavern kit is as good a place to begin as any I can think of. For filler I would use a very modest amount of Squadron putty – i usually use the green but they also have a white that takes paint well
It can be stuffed into a gap with a toothpick and when dried, shaved down with a knife or sanded down with a small emery board (nail file) or file. But I’d tinker with the ends of the kit to see if a tighter fit is possible before trying to fill large gaps.
For some forms of flash on white metal kits a variety of blades for that hobby knife, including a chisel type blade, is a good idea. I mentioned emery boards – the nice thing about them is that they can be cut to a shape with a sissors for tight situations if need be.
Back again… I also used an Xacto to clean up the castings as well. Be careful, cut away from you for those metal castings are tricky.
For gluing, I’ve used all kinds of stuff with the different parts, so whatever works for you is fine. Pinning the pieces may or may not be necessary - but of course won’t hurt.
Be careful on the painting. Automotive primer may be too thick and ruin details. I’ve always brushed the castings, and used the color paints as their own primer. Said another way, everything took two or three coats.
Woodland scenics has some nice models. But, the metal castings sure can make things difficult.
If you’re using gap-filling CA, not absolutely necessary to pin them together – IF you’re good at getting things aligned and squared by eye. If not, then getting things held in place is a better approach. One thing I’ve found handy is a small steel plate with magnets to hold stuff.
CA works well when the joints you’re making are square and have good overlap. If not, then 5-minute epoxy is a good alternative, which you do want to hold in place as it doesn’t set as fast as CA.
Some autoprimers can be too thick for model use, the sandable ones sometimes being troublesome. Recently, I’ve been using Rustoleum Light Gray Auto Primer #2081. It’s listed as sandable, gives good coverage but will go on in a light coat that doesn’t hide detail.
Metal castings are a good way to add great looking details to your layout…I will often buy certain kits just for the castings to use around the layout.
Most white metal castings need clean up. Expect it to be part of the process. Castings will vary in quality in terms of how much work needs to be done - all the way from removing a bit of flash and parting lines to massive truing and reshaping… I make an assessment of the casting and sometimes have decided not to spend the time cleaning up bad castings I will usually look for another way to fabricate or buy the part.
For windows and doors castings I will usually look to a part from Tichy or Grant line for a plastic replacement as it is hard to make a good looking door or window casting that can match the quality of Grandt/Tichy.
I use an exacto and a file for cleaning castings. I don’t bother pinning parts - the glue works fine. If you can see the parting line or flash before painting - it will be noticeable after painting. One other tip: Take the time to true up the bottom of barrels etc that want to sit flat…well worth it and nothing ruins the look quicker than leaning barrels etc. that should be square…
Go easy on the paint - thick paint coats will obscure the details…