How to prevent beading in aluminum palette
The aluminum aspect of the palette has little bearing on the problem describe, but the enamel surface is the biggest concern. Enamel, unless otherwise modified, is going to be smooth and glossy, the worst circumstance for your undesirable water beading.

You aren't far off to consider to abrade the surface, but baking soda is not particularly abrasive. It's more likely to dissolve and become useless for the purpose.

High grit count sandpaper may be faster and more effective. Consider to look for 400 or 800 or even 1200 grit sandpaper, most often called wet/dry sandpaper. Hardware stores and big box stores will usually stock such products, as will painting stores, but for a higher price.

You'd place the palette in a stream of gently flowing water and scuff the surface of the enamel while keeping the sandpaper wet. The water prevents the sanded material from clogging the grit and may perform other tasks that escape my alleged mind at the moment.

You'd be able to angle the palette to a light source and observe the extent of the abrasion while performing the scuffing. Additionally, you can observe how the water beads on the untouched area while flowing over the portions you've sanded.