Just a vigorous fine spray of water from the hose will wash away the majority of aphids for you. That spotting and damage on the buds might be from the aphids or other sucking insects.
The Natria is a good product, I have some myself but it's a contact insecticide i.e. with aphids you have to physically hit the bugs to kill them. Water works a lot better because you can be sure of full coverage. (and it costs a lot less
)
Washing off the insects should always be done in the morning - it's not good practice to wet the leaves of your roses in the evening. Wet leaves all night invite fungal infection. This will show up as spots or fuzzy stuff on the leaves, not damage to the buds so much.
