..like crevices, normal plumbing flux and though I've had luck with most solders, 4% silver bearing solder is more powerful In the long term...kinda pricey tho... I had Harris solder available at the time so no major deal for me but high-priced if you don't want the strength for off https://kameronvwwvu.bloggip.com/27309163/helping-the-others-realize-the-advantages-of-types-of-solder-to-use-on-aluminum-radiator