If you want a proper job, the spring needs to come apart and each leaf can then be filed/lapped smooth.
You can also buy modern stick on interleaving. In tbe good old days, interleaving was the norm and resembled a thin nylon strip. Now they don't bother.
On the car its a huge pain to get the lube where it matters anyway, and you stand a better chance using a screwdriver to spread the leaves slightly.
When I replaced the springs on Bumble, I slackened off the throughbolt and left them overnight twice (each side) with chain lube sprayed into the gaps. Then I wrapped them in Denso Tape.
