Aluminum Casting Surface Finish

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I have been making Aluminum Alloy Castings For various applications. The Castings are meeting my customer's requirements in form of Quality of Castings, their functional requirements such as Radiographic standards, Hydrostatic requirements, Machining Tolerances,etc.
But when it comes to Surface finish, the castings give a repulsive feeling to my customers. Generally the Castings are made in Aluminum Alloy LM25 & after Heat- Treatment, They assume Grey colour. Whereas the customer wants natural Aluminum colour which could be lustrous. Similarly, I am also making Castings in Alloy LM6. These castings are not Heat-Treated, But nevertheless the customer wants good surface finish on the Castings.
My question is how do I achieve good finish on the castings so that my customers feel attracted to my product.

PRADEEP DATAR
- HYDERABAD, A.P. , INDIA

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For Americans, LM6 is ~equivalent to A413, with ~12 wt% Si as the predominant alloying element. LM25, with 6.5-7.5 wt% Si, is similar to 356 & A356. LM25 has slightly lower limits on other alloying elements than 356, and slightly higher limits than A356.

According to Casting Kaiser Aluminum, 3rd edn. (1974), 356 & A356 "have good polishing characteristics....Anodizing appearance is fair to very good; color is gray." LM25 should be similar, with the gray due to the Si.

A413 & LM6, with about 12 wt% Si, polish poorly. Anodizing appearance is also poor. Plating gives a better appearance.

Do you solutionize the LM25 at 1000 °F (538 °C)? What is the furnace atmosphere used?
How are you currently finishing parts: mass finishing (tumbling or vibratory barrel), bead blasting, polishing..?

Ken Vlach
- Goleta, California