One way to express this could be: "We need to point out that in some of our products, although not many, the weld seams may be noticeable upon close inspection. However, they are not easily visible and can't be seen without careful examination."
Alternatively, you could say: "It should be noted that in a few of our products, the weld seams are faintly visible upon close scrutiny, but they are not apparent to the casual observer."
In English, it's important to use precise and clear language to convey your meaning accurately.
It's necessary to understand that there are always weilding joints in a small portion of the massive products. However. they can't be noticed if you don't take a close look at the products.