If the weeds are just in spots, a small tank sprayer and Weed B Gone
or similar is most effective.

Ditto that. I discovered that the hard way, by trial and error, using
the various products.

My lawn is about 2 acres. I treat it with a 1-gallon pump sprayer
using Ortho Weed-B-Gon (2,4,D). I put on a pair of rubber boots and
wander around the lawn zapping every healthy-looking dandelion I see.
You'd be surprised how gratifying this can be, and how little time it
takes to do it. I can wander the whole 2 acres in less than 45
minutes, including stopping to refill the sprayer once or twice. I do
this a couple of times a week during the peak dandelion season. This
season, I would guess I've killed 300 dandelions. The neat thing is,
you can see them withering within 24 hours. Each time you spray, there
are fewer healthy ones. Only spray the healthy ones; don't waste time
and spray on the ones that are starting to wither. Each time you
spray, it takes less time because there are fewer dandelions. Right
now, I'm looking out my front bay window and there are ZERO dandelions
to be seen anywhere.

Another benefit of this is, if you have trees, you don't have to worry
about harming them. If you read the fine print on the big bags of
"weed and feed", you'll see it tells you NOT to use the product within
the drip line of your trees. Weed killer is poisonous to trees as
well as weeds. (It probably won't outright kill an established healthy
tree, but it's not going to help it either). Using the tank sprayer to
make spot applications, I don't worry about the the trees since so
little product is used on a per-square-foot basis.