I never "grind" Engineers.
I play the game, doing what i feel like doing at the time, and if i see a material i don't recognise, I collect it. Or if i see ones marked as Rare.
Sometimes I'll just take collector limpets bounty hunting and scoop random things up.
Then if i decide to Engineers something, say a new missile rack, I'll check inara.cz (IMHO, this info should be in game FD!), and see if i can do anything.
If I'm missing something I don't often come across, like Tin for example, I will land and drive about for a bit. I enjoy it anyway, and I'll purposely pick an interesting planet and landing spot to do it from.
Last time, I landed my Anaconda on a tiny, but very tall plateau in the middle of a perfectly flat canyon, and jumped my SRV off it, just cuz'.

Even though there was plenty of room down below. Lol
15 minutes of crazy driving later, I'd collected all the Tin I needed, plus loads more materials I use for ammo synthesis.
Then off to the Engineer, sorted out my missiles, and continued my aimless wandering. Lol